When the Web was young I was on a mailing list called The Tunings List where you could discuss tunings and ask the experts. It closed and the members switched to Yahoo Groups. That too is coming to an end. So...
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I had a couple of emails recently from ex-members of the mailing-list telling me where everyone had gone. The information below is taken from those. .....................................................................................................................
Yahoo Groups! is shutting down permanently on December 15, 2020, meaning that even email functionality will be broken.
At this point, virtually all of the online microtonal/xenharmonic presence is at Facebook's "Xenharmonic Alliance," "Xenharmonic Alliance - Mathematical Theory", and "Microtonal Music and Tuning Theory" groups, all of which you can join.
The messages from this group were all archived. The archive is at: https://yahootuninggroupsultimatebackup.github.io/
You can also always get the Xenharmonic Wiki at: https://en.xen.wiki/w/Main_Page
There's also a group on MeWe https://mewe.com/group/5c9c1bc3d71e3023f90fe123
....and an email list run by Aaron Krister Johnson at [email protected]. You should be able to sign up here: http://lists.untwelve.org/listinfo.cgi/tuning-discuss-untwelve.org