I'll quote firstly from the latest issue of the Ubuntu Weekly Newsletter (issue 915)
[Updated] Questing Quokka Release Notes
The Ubuntu 25.10 (Questing Quokka) Release notes have been updated a few times since release, but we’re bringing this update to your attention. In UWN issue 913 we highlighted upgrades from 25.04 were expected on or before October 23, with that date now pushed to ‘on or before November 3, 2025’. The blocker bugs and reason for delay are also listed.
Questing Quokka Release Notes - Release - Ubuntu Community Hub
I'll also provide a link to an equivalent question I've answered on AskUbuntu as that actually has a partial quote from the Ubuntu Release Notes that give full details.
The fixes required have actually been Fixed Released if you look, however packages available in proposed aren't yet that useful for a normal install (most people don't have proposed enabled!) thus the wait until SRU delays complete.
Ubuntu [MATE] 25.10 has been released for new installs, which is what the ISO release is for, the release-upgrades don't open till later, which currently is still in the near future, delayed as Ubuntu prizes stability, and has specific Stable Release Upgrade procedures to ensure that (which do include time delays, giving those who force install time to discover problems & report them etc)
That is why the meta release file still has a "Supported: 0" preventing the upgrade from being seen; it's not yet supported.
The Ubuntu team included many announcements for the release of Ubuntu 25.10 (Desktop, Server and flavors), I know as I actually posted one of them. with a link on a thread on this site too if I recall correctly. The flavors involved in the release were all listed, and that included Ubuntu MATE 25.10.
The release-upgrade process is handled by the Ubuntu Release team, who monitor all bug reports & reported Quality Assurance & CI testing; none of the Ubuntu flavors in that decision; the best place to go is Ubuntu Release notes for the actual release (as I noted in the UWN, its updated rather regularly!)