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
- Specifying and Installing Minimalist UbuntuMATE
- Ubuntu Mate from scratch. How to?
- Ubuntu Mate 26.04 will be released?
- If LTS 26.04 will not be present, what is the best way to install the most stable "26.04" via commandline
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