A limit always exists. If you don't enforce it yourself you will find out when someone decides to send you 64GB of data to hash as their password. So always better enforce the limit yourself.
Sure could be, but to play devils advocate maybe not, some hashing libraries have limits (silent truncation or otherwise) and/or it could be reasonable not to allow users to make the backend hash strings of unbounded length.
A limit always exists. If you don't enforce it yourself you will find out when someone decides to send you 64GB of data to hash as their password. So always better enforce the limit yourself.