As detailed in Access to system header files for a strictly confined snap ccls needs access to header files and libraries etc from the host system - this is not possible via strict mode even with the use of system-files (as detailed in the thread mentioned earlier) so the only way for ccls to be properly functional is to be a classic snap. Also ccls is very similar in functionality to the clangd snap which is also similarly a classic snap.
The requirements are understood and I’ve vetted the publisher. This is now live.