I am currently working on this branch. To do so
cgit clone git://git.postgresql.org/git/postgresql.git
cd postgresql
git branch -r
git checkout -b origin/REL9_6_STABLE
9.6.17 was released in February 2020. I was a bit curious about this commit before pushing it
commit 93745f1e019543fe7b742d0c5e971aad8d08fd56
Author: Thomas Munro 
Date:   Sat Feb 1 14:29:13 2020 +1300

    Fix memory leak on DSM slot exhaustion.
    
    If we attempt to create a DSM segment when no slots are available,
    we should return the memory to the operating system.  Previously
    we did that if the DSM_CREATE_NULL_IF_MAXSEGMENTS flag was
    passed in, but we didn't do it if an error was raised.  Repair.
    
    Back-patch to 9.4, where DSM segments arrived.
    
    Author: Thomas Munro
    Reviewed-by: Robert Haas
    Reported-by: Julian Backes
    Discussion: https://postgr.es/m/CA%2BhUKGKAAoEw-R4om0d2YM4eqT1eGEi6%3DQot-3ceDR-SLiWVDw%40mail.gmail.com
Using the link above then I got all details I want.

Conclusion: Back-ported to 9.4, 9.5, 9.6.