It would be a problem, except that the behaviour you're moving away from is a stale pointer. So surely any application that'd be leaking under that new behaviour would be crashing today.
The same advice applies to a correct program regardless of the implementation: using setenv is a bug. Only use getenv.
Getenv in a program without setenv is fine in both implementations. Setenv is unusable with all conforming implementations.
To pass environments to children, use execve.
The Linux behavior allows a careful single threaded program to use setenv correctly. The BSD/Solaris behavior makes all usage incorrect, but the incorrectness comes in the form of a memory leak, which is preferable to a security issue, usually.
There's no correct, portable use of setenv. If you call it, it's a bug.
