It's typically to declare bankruptcy on the entirety of /v1 and force eventual migration of everyone onto /v2 (if that's even possible).
For example dup(), dup2(), dup3() and pipe(), pipe2() etc
LWN has an article: https://lwn.net/Articles/585415/
It talks about avoiding this by designing future APIs using a flags bitmask to allow API to be extended in future.
It's typically to declare bankruptcy on the entirety of /v1 and force eventual migration of everyone onto /v2 (if that's even possible).