I've been running GIMP 2.8 for quite a while without this problem.
Following a recent Windows 11 update (26H2, build 26300.9711),
I found that Smart App Control was blocking script-fu.exe (version 2.8.11.0) in my user plug-ins folder,
causing Script-Fu filters to disappear from their menus.

I'm not sure why that older version was in my user folder,
but it had always worked until recently.
My GIMP 2.8.18 installation has its own script-fu.exe (version 2.8.18.0) in the program's installation folder.

Renaming the older file to script-fu.exe.bak as a test.
Allowed GIMP to use the matching version and all the filters reappeared.

I can't say for certain what triggered the change.
I'm documenting this in case others encounter the same problem with GIMP 2.8 on Windows 11.