Unfortunately, even Valve is doing the bare minimum to keep TF2 up, and will probably let it go down that slow and gradual fade into obscurity, so they can continue to milk whales with more hats, and collecting Valve tax on each hat transaction. DOTA 2 and CSGO are also going down a similar path, although Valve has incentive to keep them more updated, to cash in on that sweet E-Sports money.
I don't remember if Valve has ever shut down one of their own games for good though, given that Artifact and Underlords are still playable, even though Valve said that they have given up on Artifact for good, and there are signs that they have done the same with Underlords (the end date of their Battle Pass was quietly changed from 2021, to 2031), even though they haven't said anything about giving up on that game, yet.