I will stick to Debian based distros, Arch would not be a choice for me personally. I did see Parrot (which I have used in the past) has now moved to KDE. I don’t see that as a good omen for Mate. I still have an installtion of Sparky Mate on a different laptop. But Mint Mate would seem the likeliest candidate for me if Ubuntu Mate went away. If Mate itself is the problem and goes away, I would have to consider Cinnamon and KDE. I am not a big fan of Xfce.
A quick search of DistroWatch shows 68 Operating systems now offering the Mate Destkop.
When I limited the search to just Debian stable and Ubuntu LTS it comes up with only sixteen choices. (I had to add in Ubuntu Mate as it didn’t include it in the LTS because of not meeting the 26.04 deadline).