J.R., Munich, Germany
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I provide a free non commercial garden forum (330 users).
It was a Ning2, but since Ning was bought, things went bad.
I first created a simple backup solution using D7 (did not use it),
and then a D8 version, which went live on 19 may 2016.
Both solutions migrate users, blogs, groups, discussions, event:
they are more a starter kit than might save some migration cost.
Not migrated are profile comments. For D8, Ning groups
become a site type + general forum with links to site/forum.
The migration uses bad tricks (set AUTO_INCREMENT on table
such that comments get a predefined id: required for link migration).

I published the first D8 migration (8.1.10) on 2-Oct-2016;
There remain things to be done, but nevertheless:
https://www.drupal.org/node/2810683
Meanwhile, I am on Drupal 9:
https://www.krautgarten-forum.de

I also have a D7 base version (no wistles and bells, just data),
which was used to migrate a site of 3400 users (with OG).
It's just summer, so the garden has priority over publication.

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Special thanks to larowlan and all the other contributors for fixing
a bug in Drupal 8 which broke my migrated links:
this fix made me and my forum users very happy!
https://www.drupal.org/node/2730973

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I voted!
https://assoc.drupal.org/drupal-association-board-elections