I'm hoping to ping this to the top by adding that I'm also running into this issue with a slightly different environment on a fresh install. I'm encountering it when attempting to load Module Studio.
Your Environment
SuiteCRM Version used: 8.5.1
Browser name and version (e.g. Chrome Version 51.0.2704.63 (64-bit)): Opera (Chromium v121.0.6167.160)
Environment name and version: PHP 8.0
Operating System and version: Apache 2.4.58 running on linux-x86_64
Issue
The behaviour was commited at https://github.com/salesagility/SuiteCRM-Core/commit/f01cfeff6d1f062cdd8958e666d027d0b9f6a4a2 but when calling from the User class the post_install it's not defined so the Notice happens.
Your Environment