The problem is that in Old Russian ɕ was represented by ш! Ш later became a hard consonant, moving back into mouth to /ʂ/, leaving room for a new soft /ɕ/ to be developed later. If щ was always /ɕ/, then it should also have shifted to /ʂ/, but it didn't. It must have been pronounced other than /ɕ/ at some point.Moose-tache wrote: ↑Tue Aug 25, 2020 2:21 pmThe letter щ represents ɕ, and always has; there's really no wiggle room on that point.
How do we know ш was once /ɕ/? One reason is that it is subject to the same spelling rules as soft consonants. For example, unstressed o > e after soft consonants, though this also includes after hard ш (and ж as well, which likewise switched from soft to hard). Hence spellings like хорoшего rather than хорошого. The ш in this word is hard today.
Source: Tore Nesset, How Russian Came to Be the Way It Is (2015), which is a great book for linguistically-minded students of Russian. Your post set me to browsing it again.
EDIT: And here are some shots of the pages where Nesset discusses the rise of /ɕ/ from /ɕtɕ/ (though he spells it using a non-IPA system particular to Slavic linguistics). There appears to be no controversy among slavicists over the source of the sound. The Latin transcription "shch" indeed is reflecting an older pronunciation.