Right now, the l= tag is required with at least one URI in a BIMI Assertion Record. However, an empty z= tag is an explicit declination to use BIMI for mail using that selector, for which there is no reasonable l= value.
There should be an explicit record for "do not use BIMI" that looks like "v=BIMI1; z=; l=;" or "v=BIMI1; z=" stated explicitly and the ABNF for this should be updated to match.
Right now, the l= tag is required with at least one URI in a BIMI Assertion Record. However, an empty z= tag is an explicit declination to use BIMI for mail using that selector, for which there is no reasonable l= value.
There should be an explicit record for "do not use BIMI" that looks like "v=BIMI1; z=; l=;" or "v=BIMI1; z=" stated explicitly and the ABNF for this should be updated to match.