There are chances that Ubuntu Mate as an Ubuntu 'flavour' can cease to exist. Some UM community members say that they are inclined to move to another distro be that the case.
IMHO, migration to mainstream Ubuntu desktop + Mate DE is another choice which is worth some consideration. At least I'd propose to try that first because my personal experience from mainstream Ubuntu desktop and manually installed Mate DE is positive.
I expect Ubuntu + MATE desktop would work OK.
I've used Debian + MATE desktop, I added the themes from UM and it's kinda close? Still not as clean as the UM experience is (or was). And the Debian installer is less user friendly.
There's lots of options to explore.Ideally I would like to stick with Ubuntu so one of the options is to use Ubuntu Server + MATE DE. Also looking at trying Ubuntu Cinnamon and Xubuntu. Outside of Ubuntu Mint MATE would be a top choice.
Still have a year to experiment and I have a spare laptop to experiment with while keeping my UM laptop as my daily driver.
That's a popular opinion which looks somewhat doubtful to me. You see, server installation differs from desktop one. At least it was different some years ago. Say, it had no USB stick automount feature and Ubuntu server did not automatically discover and configure wi-fi cards. That was aligned with server security considerations. That is not a big deal for server guys but may be a problem for an average desktop user.
There is definitely lots to consider before making a final decision.I still have a year to explore my options and with the spare laptop I can experiment without worry.
Ideally it would be awesome if the right folks came along and UM rises up to it's full potential again. 28.04 is two years away and a lot can happen in that time.
Experiment is the keyword here. Also a lot can happen in a year from now and it’s good to explore your options just in case. I only have one desktop computer that is around 13 years old, so I have to try everything on it.
During the past 10 or 12 months I did try different setups, as I love Ubuntu MATE but was experiencing a few issues and I couldn’t find a fix for them or couldn’t get them to work the way I needed.
So I tried Ubuntu as the base system, either with xubuntu-desktop or mate-desktop on top, all good but a few little things did not work the way I needed (this always happes to me when I try Ubuntu, either with its default DE or with a 2nd DE). I didn’t venture into Ubuntu Server as it’s way out of my scope.
Tried Xubuntu as my base system, as it always worked fine on me, never a single issue. Then installed MATE DE on top of it. Worked great, just some issues because of my hardware. But everything worked the way I needed.
Weeks ago I switched to Linux Mint as my base OS (which is based on Ubuntu), went for the XFCE edition and works fine, I know they’ve got a MATE edition, but as I don’t want to swipe everything to install a new system, and installing the two systems as dual boot would take up much disk space, I will install MATE as a secondary DE and switch to it. I’ve been doing some research and I know what to install and what not to make it as clean as possible.
I learned quite a lot by doing all of this.
I’ve done this for a friend, it worked OK putting Mate on top of Unity for what I believe was 20.04, but the default apps weren’t always what I expected or wanted, since he was a newbie and somewhat happy with Unity, not as much of an issue for him as it was for me when I was using his system teaching him my AI Serurity Camera Addon system.
I've talked about this many times before in various threads, so I won't repeat most of that.
When I started using Ubuntu & flavors I had bandwidth restrictions at home, so I'm very used to installing one system (eg. Ubuntu Desktop) and switching that to the flavor I wanted to try via package changes; as I could do that bandwidth [quota] free.
If I wanted a Ubuntu MATE install & there was no ISO, I'd just use a Ubuntu Server install in most cases, and then add ubuntu-mate-desktop on top of that. Of course I'd consider release & may even do some homework before doing it (ie. contrasting seed files which control what goes on an ISO, and comparing other releases for differences).
I know ubuntu-desktop-installer requires format of / (since 24.04), but as I'm not using subiquity enough to know if it does I'd check that wasn't an issue for me. If it was I might also start with an ISO that uses the calamares installer, as it allows install without format of /, then remove the flavor desktop that was installed, and add ubuntu-mate-desktop package.
In the prior posts I've written about the Debian+Ubuntu MATE packaging team which may package now with regards only for Debian's release dates, if the Ubuntu part of that equation isn't there (ie. no Ubuntu MATE developers).. but that's a timing issue I've discussed before elsewhere, and most just a consideration.
Myself, I started using Debian years before the Ubuntu project even started (which was 2004) and I've never stopped using Debian, so I'd still be happy there. I'm mostly using Ubuntu (85%+ of the time) as I do find it easier for desktops; and I can't see anything currently which I'd miss if Ubuntu MATE didn't release ISOs as I'm usually installing using other media anyway; as I like my multi-desktop installs (which I've written about elsewhere too).
Other links that may be useful include
The future is still being written, Ubuntu MATE is still an official flavor of Ubuntu so nothing thus far has changed, none of us know what will change though, and Ubuntu 26.10 & later could be somewhat different to what I understand anyway.
there was one link I really wanted to add, but I couldn't find it, thus you've got a few that talk around the topic
This was in the OMG! Ubuntu article,it's something to consider.
Flavours can still release a non-LTS during an LTS cycle. The difference in what means arguably depends on what its users expect: it will get underlying updates and security patches for the duration of the LTS cycle, but flavour-specific desktop apps and packages may not.
I think that's already been covered; I used Ubuntu Studio 18.04 as my example, of that, with the Ubuntu Studio team providing updates via PPA, and I even covered the release-upgrade process (as Ubuntu Studio 20.04 LTS was a LTS).
I also addressed where Ubuntu MATE stands in relation to coming Ubuntu 26.04 release (ie. the missed mandatory flavor requirement that makes release itself difficult).
Aren't we just going in circles on the various threads on this site?
I guess but that one paragraph I posted simplifies it for the average end user compared to the entire large multi-paragraphed replies already posted.Might as well make the options as easy to understand as possible.
But since this site isn't going to be here much longer does one more post really matter anymore?
Anyone watching the Ubuntu MATE Development Matrix communications channel will see this future playing out... ie. keys of the kingdom being handed over to new stewards.
I'll also quote from Ubuntu Discourse
For those who 
Ubuntu MATE, that has to be great news I feel.
I think to many people are leaving way to early but I can understand that because of the uncertainty.
I will stick around, but keep a current OS of Mint Mate burnt to a USB drive. I have had OS’s shut down on me before, Ikey’s first two Solus OS’s and Point Linux. It sucks but after the first time I keep myself prepared so it’s not so bad. Also if you have an old laptop keep a current edition of something else on that. I have Sparky an old laptop that runs just fine.
If I can avoid the horrible ubuntu installer, I will.
I even might switch to Lubuntu/Kubuntu (calamares)
If push comes to shove and things like 'snap' become a hard dependency instead of a choice, I will jump ship to either Debian+MATE, Garuda Linux or Mint.
Another one on my list is MX-linux , they even have the Moksha desktop as option 
Mint or Mint DE are both great choices. They look the same with a few differences under the hood. I used the same desktop set up and wallpaper and you couldn’t tell the differnce. Clem also avoids snaps.
I used Mint for over a year and basically left because this was the better forum. which will soon no longer exist.