After having been called forum of the week on the 23rd May, I might announce it here now as well.
We are quite an old German speaking C++ community on the internet. It goes back to the 1997/98. The previous forum was opened in 2001 and was a phpBB2. We expanded and maintained that phpBB2 ourselves for 17 years until I finally had enough of the horrible code and design of 2001. I wrote my own plugin and migrated with a modified NodeBB. We took over post IDs and topic IDs, converted all BBCode (including our custom extensions) over to Markdown and thus took all the data with us. This means about 2.25 millions posts distributed over about 280,000 topics with nearly 35,000 users. The migrations plugin took 15-16 hours to convert all this data.
So far quite happy with the new forum. It opens up a lot of new possibilities for us. Also helps with search indexes since it is now mobile friendly. Also it is now mobile friendly . I think the majority of the community is also happy with the modernization and new users to the site will probably be more familiar with such a modern design and with Markdown.
We are still missing some stuff from the previous forum and have some open wishes for additional features or improvements. But I will work on this stuff and hopefully share it via core contributions or plugins with the community of NodeBB.