That is Ubuntu 26.04 LTS, which currently sits in Release Candidate status prior to actual ISO release.
Ubuntu MATE 26.04 is NOT expected to release for new installs, as the prior lead developer has stood down, but that was done too late for newer developers to actually reach release for 26.04; a required milestone already having been missed.
The issue that stopped builds back after March 4 2026 had fix(es) submitted by a Ubuntu community member(s), but as no Ubuntu MATE developer reviewed/accepted it; it sat for a long time before being unapproved; ie. the failure was mostly personnel lacking time, or motivation as outlined earlier. Software changes over time, Ubuntu has flavor sync meetings so flavor teams can keep in sync with everyone else.
In answering a support question about resolute I used a recent QA test install of a resolute ISO to switch from one flavor to Ubuntu Desktop, then later did it again switching to Ubuntu MATE which I mentioned elsewhere where the I was able to login and use Ubuntu MATE as stated in that post. Ubuntu MATE 26.04 looked good to me; lack of new wallpapers etc not a problem for me.
A release-upgrade, as I tried to outline in my prior post on this thread, will see your system as a Ubuntu 25.10 system, and upgrade that to 26.04, which includes all packages; thus upgrading the MATE component as well.
I have not done a release-upgrade test from Ubuntu MATE 25.10 to 26.04. My using a recent QA test install (so no setup required at all) for what really was just some package changes whilst I was doing my work on another box required only minutes of my actual time (to issue commands, sure it took more than an hour of execution time due to my using an old Core2Duo box, spinning rust drive & with my slow aussie internet), but cost to me was mostly electricity and not time.
If you’ve got capacity, you could install and run your own QA test of an upgrade; ie. install Ubuntu MATE 25.10, fully upgrade it, then do a -d release-upgrade test yourself. You can report results here too (this thread would make sense to me).
If you’re wanting something from the new Ubuntu MATE development team, I’ll suggest giving them some time to work out their plans, which will be I’m betting on the next release; ie. Ubuntu MATE 26.10, since it was already too late (my opinion!) when the prior lead stepped down for Ubuntu MATE 26.04 to occur; alas as the Ubuntu MATE development team was so small, there weren’t others around to ‘pick up the slack’ for the 26.04 release.