What is the best identifier for a UserID?  64 bit integers, UUID V5,  or 64 character SHA256 UID?
Use integers. This is the simplest and most efficient way (especially for InnoDB).

That is


don't use hashes or UUIDs or varchars in place of int
don't use the natural key when you have foreign keys to it


Reasons?


varchar comparisons require collation + case tests
index architecture in InnoDB favours integer fields (ideal clustered index is narrow, numeric, monotonically increasing)
hashes and GUIDs and varchar cause fragmentation
wider data + indexes rows when not using integer
64 characters requires 64/66 bytes to store. 64 bit in requires 8 bytes


Other observations:


Do you need 64 bit numbers?