There's no explicit rule but given "the guidelines just say fesco approves changes for marking packages as deprecated, not who can approve exceptions to it" ... we'd tend to lean towards fesco approving exceptions unless they say otherwise.

You probably also want to lead with this fact in your fesco ticket:

Notice: this extension was part of php-src (provided by php-pspell package) until 8.3, was removed in 8.4.

FWIW we do have a package split exception rule already ... but it's to the "no new package can use the CC0-1.0 license for code". Nobody in the committee can think of a reason why that shouldn't be a generic exception (package splits aren't classified as a new package) ... so if someone wanted to do a PR adding that to the packaging docs. then when it's passed you wouldn't need this, or a fesco, ticket.