The only known limitatation is that user logins are still max length 25
Display names can in theory be longer in length (since they can use non latin characters and how your system counts will depend on your system)
ID’s for things will just grow, whether thay be numeric ID’s (video ID) or string ID’s (user ID’s are strings)
Most other things people don’t store/refetch from the API, just cache so a rolling cache is gonna roll anyway.