How to deal with deprecated / unsupported GDAL drivers
GDAL now maintains a separate project with unsupported drivers: https://github.com/OSGeo/gdal-extra-drivers
With the new release of GDAL 3.3.0 more drivers are marked as deprecated: https://github.com/OSGeo/gdal/releases/tag/v3.3.0
The following drivers included in Fiona are already moved to gdal-extra-drivers with GDAL 3.3: AeronavFAA, BNA, SEGY, SUA.
With GDAL 3.5, the following drivers will be removed from GDAL (and probably moved to gdal-extra-drivers): ARCGEN, GTM=GPSTrackMaker?
I assume most of these drivers can be considered "exotic". Thus I would suggest removing them from Fiona supported driver list with the first release of Fiona 1.9.x.
Also, this would be a good occasion to review the list of GDALs currently supported drivers to check if new drivers should be supported.