Memory footprint diff between 9.5 and 12

I hadn't noticed this until today, but a running 9.5 system with buffers at
10GB starts and has been running years without issues. (15GB available)

Postgres 12 will not start with that configuration, complaining about
memory availability. So Postgres12 won't start until shared buffers is
6GB, but even with that, my DB servers , postgres queries started
complaining about being unable to allocate memory "unable to allocate".

So dropping them to 4GB (on a 15GB system), may help, but did I miss a huge
note about significant memory changes between 9.5 to 12?

Is there something else I'm missing that on busy systems is important,
something introduced in 10 or 11 as again I'm not seeing anything noted in
12.
Thanks
Tory