7.0.1 got a second chance, and lo and behold: I found and fixed the issue.The problem is in the
$HOME/.config/kicad/7.0/sym-lib-table
configuration file.
This file is generated during installation with brute force, regardless of whether you choose the “standard” or “custom” tick boxes on startup.
The old
$HOME/.config/kicad/6.0/sym-lib-table
configuration file is not considered at all, and thus the custom libraries disappear from the symbol list. The libraries themselves are still there >wipes brow and takes a deep breath<.
There is no function in 7.0.1 to correct this or to import the old symbol table.
Result: your custom libraries are gone, and you have to add each one manually. Not good, and unnecessary.
The way I found around this was to edit the 7.0/sym-lib-table manually and insert the missing lines from the 6.0/sym-lib-table.
I consider this a bug, or at least user-unfriendly programming.