I noticed the same thing recently; no real warning once a transition is made to snap (there possibly is some output from whichever sudo apt update corresponded with the changeover).
The old core snaps tend to stick around even after all installed packages have moved on to other cores.
You can look up which snaps are dependent on others via snap connections. However, this won't indicate which base a snap uses (e.g. chromium is based on core22). The base for a snap can be seen with snap info --verbose.
so a 24.10 system which has only firefoxsnap package added will require those so firefox can run.
A quick look and 22.04 firefox has those same requirements as well.
Ubuntu MATE 22.04 is now End of Life, as being a flavor of Ubuntu Desktop, it came with only 3 years of supported life. This applies to all 22.04 flavors and not just Ubuntu-MATE.
Do note: If you have multiple snap packages that were compiled on different releases; you'll need a Core package for each release the systems were built on; ie. for both 24.10 & 22.04 I've provided here, as the package was built on 22.04 they'll require Core22, but different snap packages may require different Core packages (eg. an older package built on 18.04 may require Core18, not being able to use a Core22 you may already have installed).
My habit, after one bad experience, is to only move from one LTS to the next LTS which is 2 full releases behind. In other words, when 26.04 roll out, I will upgrade to 24.04.5 .
That approach has given me the best experience and stability. When I am "desperate", I upgrade individual Application packages directly, bypassing the distro's "integrated" versions.
Also, I have forced Firefox to non-snap, so I don't see why those packages would download, if I forced Debian-style packaging for it. Since I have removed SNAP-Firefox, can I remove the gnome snaps*, without affecting any other Applications?
As already stated, Ubuntu-MATE 22.04 is End of Life, as are all flavors, thus I have no intention supporting that release on a flavor site, as that goes against what unsupported means.
A quick look at a manifest file will show you what is included on an ISO, and thus indicates what can be expected on a new install (though options made in installer can change that, with options to download & install additional packages no on ISO, and minimal options to cause some packages to not install or be erased)
If you have more than those, it's likely the result of other deb packages you have installed. Don't forget if a browser for example is required as a deb package; it won't be coded as any web browser from any source, but specific deb package(s) available in the Ubuntu repository, and those packages can cause other deb packages to be required, with yes some being stubs that can install snap packages.
Currently supported releases of Ubuntu-MATE include
The second column essentially says that on my system, firefox, firmware-updater, snap-store, and snapd-desktop-integration all depend on gnome-42-2204.
You can do that for any snap you want to remove. I'd also check the base snap for each installed snap, something like this should provide a list:
snap list | \
sed -r 's/^([^[:space:]]+)[[:space:]]+[0-9].*/\1/g' | \
xargs snap info --verbose | \
grep '^base:'
So if core20 was sitting on my system (which it was a couple of weeks back), I could safely remove it (snap remove core20) because it did not come up in the connections, nor was it the base for any of my installed snaps.
