@burin077
I found this description of the Dec 17, 2015 patch but do not see specifics about relationship culling, just that they found that "it turns out culling happens because of the need of those three items: load times, memory usage, and performance." https://simsvip.com/2015/12/04/the-sims-4-new-game-patch-december-4th-2015/
This thread has some good information about relationship culling that you may find helpful:
https://forums.thesims.com/en_US/discussion/comment/15366829#Comment_15366829
@Neia , do you know of any specifics on relationship culling for the game currently?