I realized I didn't actually open an issue for the #3206 PR I opened,

The original issue was that CRS.to_authority function only has a few hardcoded authorities for CRS's that are accepted: EPSG, OGC and ESRI in that order via a if/elif/else block.

However there are a large number of CRSs from the IAU authority for planetary bodies that have been supported in Proj for a while now, and are accessible using CRS.from_user_input. But calling to_authority on these would result in a None return.

The PR (#3206) fixes this by adding IAU_2015 as an authority to the if/elif/else block and adds two tests. Confusingly, while IAU is the true name of the authority, I believe the 2015 postfix is simply the version number, why this is attached to the authority name is not clear to me.

I did notice however that it would be possible to simplify the function further to not bother testing the authority name order by doing the following:

if not (matches := self._matches(confidence_threshold=confidence_threshold)):
    return None
authority, code = next(iter(matches.items()))
return authority, code[0]

but this comment block seems to indicate it was desirable to always prioritize the EPSG:

        # Note: before version 1.2.7 this function only paid attention
        # to EPSG as an authority, which is why it takes priority over
        # others even if they were a better match.

so the above simplification would not be valid, but if this preference for EPSG is no longer necessary I can update the PR to use the new method which will simply return the authority that best matches the CRS