Under what time frame does the list think it would be acceptable to take over the package? And additionally, are there any other interested volunteers to co-maintain? (I do not want to be the main maintainer, due to more direct interests in other topics.)

If you do not want to be the main maintainer and do not have much interest in cryptography, then it sounds wrong to initiate a takeover. There is a procedure for non-maintainer uploads via Hackage Trustees, please file an issue at https://github.com/haskell-infra/hackage-trustees/issues, linking a patched repository.

I cannot switch my main GHC version to 9.0 at this moment.

It would be nice to audit that `cryptonite` is not vulnerable to https://mail.haskell.org/pipermail/haskell-cafe/2021-March/133540.html

Best regards,

Andrew