Message signatures, structured fields and ABNF

Dear all,

after reading the HTTP Style guide
https://httpwg.org/admin/editors/style-guide#self-references
I was trying to refactor RateLimit
https://github.com/ietf-wg-httpapi/ratelimit-headers/blob/main/draft-ietf-httpapi-ratelimit-headers.md
to remove ABNF and I am finding the document less readable than before.

Looking at https://github.com/httpwg/http-extensions/blob/main/draft-ietf-httpbis-message-signatures.md#creating-the-signature-base-create-sig-input
I think that removing ABNF from there is even more complicated.

Which is a suitable replacement for ABNF to be used in cases such as
Message Signatures?
While Structured Fields is capable of disassembling the structure of  a field,
it can't formally describe the specific content/format of a string.

Kind regards,
R.