I'm not aware of Steam ever having removed a purchased game from a library.
If you have a reliable source that says otherwise please link it.

Technically they could do that of course.
But it would be bad for their business and have people do more business with gog.com.
Steam gets chosen over gog.com for convenience.
If purchased games disappear that will be even more inconvenient.

edit: I mean "Steam ever having removed a purchased game from a library" in the sense of for no good reason.
Following legal requirements or being dependent on external resources (e.g. game servers) outside the responsibility of Steam are different things and I wouldn't blame Steam for it.
There's a reason for the trust people have in Steam.
And there are plenty of reasons for not having trust in Sony.