Is it possible to run mineOS on termux (root possible)?
MineOS maybe, java minecraft - probably not.
There may not be a way to access any services even if you did manage to install it and run a server.
If you give it a go please let us know how you get along.
What instructions shold I use? Debian?
And I did manage to get a normal java server running (and working) and can play java Minecraft on my phone (pojav launcher).
Possibly debian due to apt package manager but afaik this would be new territory and I don’t have android to really try or assist.
First time of hearing of Termux but to me it sounds like a virtualization layer sort of like virtualbox but for phones. assuming you can install what you need to in there and there is still a mineos for arm then sure.
I guess you will have to try and see tbh most people here probably are not using phones to host Minecraft servers i mean not trying to shoot you down just that its probably not common so just try and see what you can do i guess and maybe someone will come in that might know.