I can't think of a single Leica M film camera (except the M7) that can't still be repaired/CLA'd - even the M5 and CL, but IIRC Leica wont touch anything older than an M4-2.
There are now no replacement circuit boards available for the original M6 classic or TTL. In this case, if your meter dies, your M6 will become a glorified M4-P.
Leicaflex cameras can still be CLA'd - but AFAIK, Leica won't touch them.
AFAIK - Digital Ms can be repaired back to the M240 series, but only thru Leica.
Any Leica R film camera? - Forget about it!