I’m asking because I want to play classics like Half-Life and such, legit (like not downloading cracked versions that might contain viruses).
Hi, if you have your key, you are better off grabbing the ISO from archive.org ( Half Life (Sierra)(Valve)(1998) : Free Download, Borrow, and Streaming : Internet Archive ) , and installing that way for the classic -non-steam version, install the HL1110.exe (~84mb) update as well and enjoy 
managing steam issued game keys:
Steam Support :: Steam-Issued Activation Keys.
Unfortunately you won’t be able to install Steam in 2023, but you can extract your installed Half Life from your computer, and transfer to your Windows XP computer and run it
you can work around it with this, not sure if it works, content servers can be unavailable
Oh, nice!
It seems like there is another edit on the post with a fix to it
Maybe I could try it on Windows Vista but idk abt XP
Hm, nice
This doesn’t work no more, they butchered the servers 
Thank you! Will try!