Question about Steam games not on the Steam store

They are free on Rockstar website.

They could never be purchased separately anyway, just in the series collection. They are still included in that, last I heard, they just no longer have store pages.

I'm pretty sure they still come in the GTA Complete Pack (like so, though I'm not sure if this is the one still on the store), but you should check the forums to be certain (they might be in other packs too -- I didn't really check).