Yes. I'd say any 25+-year-old game where 99% of gameplay knowledge is transferrable and they still have a large playerbase fits the bill for "immortal" games.

Yes. I'd say any 25+-year-old game where 99% of gameplay knowledge is transferrable and they still have a large playerbase fits the bill for "immortal" games. There are opensource reimplementations of many of such classics anyways. Civ games definitely pass the test. The classic Doom style boomer-shooter, the mario-style platformer, the Arena-style Quake3 shooters also have many opensource reimplementations. Some may not be rule-for-rule bug-compatible reimplementations, but they're definitely "close-enough" variants of the game that scratch the same itches and could be picked up quickly by a player who enjoys the classic. What's more surprising are the giants of gaming that don't get this kind of treatment. Like, there are reimplementations of classic RTS games like Warcraft 2 and Red Alert, but the but the art in these games are all asset-rips. You don't have something like Xonotic or Unciv that rebuilds the game fully from scratch. The only RTS game to get that treatment AFAIK is Total Annihilation thanks to Zero-K and Beyond All Reason.