As per Stas' request in projectfluent/fluent.js#84 (comment)

This is one of those things that I don't think developers will actively ask us to introduce but they might
appreciate it if it's there. If you feel strongly about forbidding the question mark (at least right now) and my
above arguments haven't convinced you otherwise, please file a new issue in the Fluent Syntax repo.

I am not convinced and I would see a value in not adding this sygil at this point.

My reasoning boils down to two items:

1. I think of l10n-id as an identifier, not a quasi representation of the english sentence (much like Axel's point in the linked discussion). Dashes stretch it, question mark breaks it.
2. Adding sygils is irreverisble and in result should be avoided when possible. There's no request for it, there's no parity with other systems issue, there's no prior experience of people using such a thing and overall, trying to convey the role of the phrase in via the "?" in the ID seems to me like a misplaced goal.

I'd love to use as few sygils in the ID as possible and the [a-Z_-0-9] is imho the far end of what we should start with.