Shameless plug: My DMARC Checker at https://dmarcchecker.app/ displays a warning message if it encounters a DKIM signature header with an 'l=' tag:
"The 'l=' tag limits how many bytes of the email body are included in the body hash. This may allow an attacker to alter/expand the message in a way that it still passes DKIM validation."
Additionally, the tool alerts you to the use of weak RSA keys or SHA1.
By the way, less than 0.4% of all emails checked make use of the 'l=' tag.
"The 'l=' tag limits how many bytes of the email body are included in the body hash. This may allow an attacker to alter/expand the message in a way that it still passes DKIM validation."
Additionally, the tool alerts you to the use of weak RSA keys or SHA1.
By the way, less than 0.4% of all emails checked make use of the 'l=' tag.