Although this is not specific to Sigil, there is no forum specific to Accessibility in Epubs. But, because of the indispensable Access-Aide plugin, I would expect that everyone with an interest in making Accessible epubs, also uses Sigil??
This is just an FYI about a really comprehensive website
https://apln.ca dedicated to helping publishers in Canada create accessible content and build a community around accessible publishing knowledge and practices.
It's become a place I visit several times each week, especially since the latest versions of InDesign have added many new features that support greater accessibility in epubs. APLN is helping to understand how best to use this new support.
Jim