Dropbox's "unlimited storage" was only truly unlimited for a very small period of time. By the time I joined in 2015, they had already changed the wording to "as much space as you need" and included concrete limits in the agreement unless customers requests quota increases.

A story I head from my time at the company: MIT was one of Dropbox's first big enterprise deals. Also significant is that both founders are MIT alums. This deal was in the era of truly "unlimited storage". One computer science professor decided to actually put this claim to the test. He wrote a program to upload pseudorandom bytes to Dropbox and periodically download segments of this data and hash it to validate integrity. I head that the amount of data was uploaded was in the petabytes range. He had specific database shards dedicated just to his user. IIRC they rate limited him, with the rationale that they agreed to unlimited storage but not an unconstrained upload rate. Eventually they got the MIT prof to put a stop to this, and I bet this was a significant in changing the wording to "as much space as you need."