The GTA performance bug didn't get fixed because back when they were trying to make the game performant, there was no problem. They kept adding new stuff to the online part of the game, and the file being parsed grew to be ridiculously large. The person who fixed the bug mentioned "63k item entries": https://nee.lv/2021/02/28/How-I-cut-GTA-Online-loading-times...
The economics of these games requires that you remove most of the development staff after launch. The games cost hundreds of millions to develop. Everyone went off to go work on GTA6 or some other title.