"Are RP-servers allowed?" a fan asked Rockstar Support. The answer he received was quite unusual. As it appears, the company doesn't mind custom RP-servers as well as the community's creativity. Though there are several restrictions. It mustn't have anything related to Rockstar Games' intellectual property.
Take-Two will definitely target at:
Unauthorised use of Rockstar Games' trademarks and Intellectual Property;
That also includes import and unauthorised use of any other companies' IP, such as brands, characters, trademarks or music;
Commercial use of the community's products (lootboxes, in-game advertisement integrations, NFT, etc.);
Custom stories, missions or maps, any actions that may affect the official online services.
Also Rockstar Support noted that it's up to Take-Two to decide what is appropriate or not when it comes to modifications and custom RP-servers. The company will demand to delete anything that might violate their rights.