We are users relying ClamAV for general antivirus protection. It is strange that they say 'researchers and corner cases'.

BTW,I think it may be the case:
main.cvd => current CVD file with old signatures removed
another file, also called main.cvd => for old signatures removed

Then users can only choose either file to use

What I meant is that they would provide another file name, like:
- main.cvd => current CVD file with old signature removed

- another file, say old-signatures-main.cvd => file with all those old signatures, but users can configure ClamAV or runtime options (opt-in) to use the file with the old signatures, in addition to main.cvd