Not that it bothers me, but is there any technical reason for said restriction?
Topic on Extension talk:Cite
As far as I can tell from the code, the reason is that refs without a name are automatically numbered, starting from 1. We want to avoid confusion between such automatically numbered and manually named refs.
An additional argument might have been that the (classic) HTML and CSS standards don't allow names, ids and such to start with a number either. It's typically a good idea to follow well-established standards.
Sample from 06:32, 19 August 2022 (UTC) <ref name= a12345 >test</ref> </references> --Ancheta Wis (talk) 06:32, 19 August 2022 (UTC)