Yes, RDS still works great at 100 GB of data, for 99% of applications. I used to generally never advise to shard until 1 TB or really think about it until about 500 GB. From dealing with customers that have sharded as early as 50 GB of data and as late as 3 TB of data, those that do shard earlier always have a smoother time.

The hard question is if you actually need to, if your data and indexes stay in cache then there, of course, is never a reason it's a matter of if you predictably know you'll grow and need to. Often for various B2B products that are about to sign up a large customer that will guarantee this growth ahead of time you can plan for this which makes life a bit easier. As a rule of thumb I wouldn't think about it before 100 GB these days, and once you hit that point I'd start to look at what your growth pattern is and at least have a plan if you expect to outgrow a single node.