Just noticed that in var/lib/boinc-client/ cc_config.xml is a link to file in /etc/boinc-client/

I am pretty sure the file itself used to be in /var/lib/boinc-client/
Curious about when it changed.

It is probably three or four years since I last manually edited the file before today so clearly I missed it.

I'm a pretty recent explorer in the world of Linux, but I've always assumed that it's a by-product of the security protocols coupled to the systemd installation model. BOINC is pretty tightly sandboxed in that scenario - BOINC programs can't write out to files outside it's private playpen, and applications launched by any other user can't modify files inside it. My installations have, I think, four user-configurable files accessed via that sort of wormhole into the shared world of /etc/.