That has been written about rather extensively, esp. on the now closed Ubuntu MATE Discourse, and is referred to at the top of this thread too, look at my reply for some basic links.
Those involved or watching the Ubuntu MATE 25.10 release noted that required milestones were missed by the development team, but that was overlooked so they could actually release a 25.10 system, as that is a milestone to have a 26.04 LTS release. Required milestones in the 26.04 were missed; with the Ubuntu MATE development team not even applying for LTS status for their 26.04 release, and then missing mandatory milestones that prevented any release of a 26.04 system.
The prior Ubuntu MATE lead wrote about it, seeking a new development team on the following thread
Whilst that was too late in the resolute or 26.04 cycle to allow for 26.04 release, it has led to a team which are working towards a stonking or Ubuntu MATE 26.10 release.
Even if you’d not been watching the progress over at Ubuntu MATE’s discourse, questions have been asked and answered at other sites too, eg. https://askubuntu.com/questions/1566241/what-is-the-current-state-of-ubuntu-mate-26-04/1566245#1566245 where I include a link by Thomas Ward of this site too, ie. https://askubuntu.com/questions/1565457/ubuntu-mate-24-04-help-for-understanding-no-26-04-lts-version-will-be-release/1565468#1565468 so for those watching, evidence was there, even if not really ‘out in the open’ or obvious.
Ubuntu MATE is still an official flavor of Ubuntu Desktop; they’ve not been removed from Ubuntu flavors | Ubuntu you’ll note; if you look you can see the resolute or 26.04(.1) ISO builds weren’t even stopped (do NOT expect it to be released!); alas the failure that hit the team back in March is still preventing those ISOs from building so they’re useless to anyone except the reports being generated (I only know about that as I mentioned it recently in a development matrix channel(s), and had accidentally used resolute in my URL but hadn’t noticed; where the link I was trying to use was stonking (26.10) in reply to new Ubuntu MATE contributor).
If you look, evidence clearly shows Ubuntu MATE as a flavor still exists; they just did NOT have a 26.04 release, as the old Ubuntu MATE development team stepped down too late, and as such the new team is aiming for Ubuntu MATE 26.10. Take a peek in the development channel (on matrix) and you’ll see activity towards 26.10, even if ISOs aren’t fixed yet.