Slitherine/Matrix removes Steam game after 6 years

Contact them about it. It has to be a mistake.

to be fair, its not steam's fault
the publisher who sold you the key told them to do so
so try to contact them

but i get your point, and so far steam is doing ok to let you keep your games
and thats why i like GOG DRM free downloads

I bought a lot of their game in steam store and some at gamersgate so far good. Hope that this not gonna happen to me.
You can try to contact Slitherine. I think they can solve it for you.
First you can go to matrix store, login your account, see whether you still can download that game's exe file or not, then you show them about this. I think is their system error. That is no way for payment problem that only happen after you bought it for so many year. That money already in their pocket long time ago.
If that is payment problem, that time already will show it to you rather than let you play for so many year.

Don't understand why.