Every Plasma application I run starts with "Failed to establish shared memory mapping, will fallback to private memory -- memory usage will increase" which sounds bad enough to investigate.
I couldn't find any help regarding this error, how to enable shared memory (other than what I have done) and why my shared memory is failing.

I have /dev/shm mounted with tempfs:
tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)

And kernel.shmmax is > 0
# /sbin/sysctl kernel.shmmax
kernel.shmmax = 2147483648

What am I missing? Why do I see this warning?

rename .kde and .config folders in your home directory to check if it solves the issue.

Hmm... I created a new user since I didn't feel like messing with my carefully built up config and indeed, in this account the problem does not exist!
So, what KDE/Plasma option could possibly be responsible for failing shared memory initialisation?

Solved the problem by recreating my home folder and only copying documents, .kde .config and .local back from old home. Guess the offending shm config was somewhere hidden in old configuration file that I can't possibly remember?