Your only options are 2K21 and 2K22.
Those of you looking to pick up NBA 2k18 for Christmas on Steam this year are going to be in for a nasty surprise; the game isn’t on sale anymore. Earlier tonight 2K Games took the move to delist three entries in the popular basketball game from sale, including their DLC, premium currency, and deluxe edition content.
NBA 2K18 carries a 29% mostly negative rating with a 24 hour peak of 293 concurrent players. 2K20 meanwhile is significantly more populated with a 51% mixed rating and a 24 hour peak of 4,862 players. The store page for NBA 2K20 includes an addendum that the servers will shut down on December 31.
NOTE: All multiplayer servers for NBA 2K20 will be shutdown as of 12/31/2021. After that time, all game functions requiring online servers will no longer function.
NBA 2K19 is still selling virtual currency through Steam and have not removed those from sale, although a 2K support page seems to suggest those should have been removed from sale well over a year ago. The servers for NBA 2K18 shut down in January of 2020. NBA 2K19 servers shut down in December of the same year. A news post on 2K’s website mentions the removal from stores on January 1, and it looks like Steam just got the cut a few weeks early.
If you were looking to pick up NBA 2K18 through 20 for the single player and absolutely nothing else, you have a couple more weeks to do so on console before they should be gone for good. Presumably this covers all platforms.