bbpress is compatible with 5.0, unless you know of any issues.
I am just a user of bbpress who helps out here, but version 2.6 has been in development for some time.
The software is very stable, and the product is not fading into oblivion, in fact it has more users now than ever.
I’ve found no issues in relation to WordPress 5.0
And BBPRESS continues to be stable but as an actively maintained and developed platform BBPRESS is dead.
As you have seen Version 2.6 has been in development for about 2 years and no updates have been made on the latest release candidate in over a year I think.
The only person providing any support here seems to be Robin, who I understand Is not part of the BBPRESS team but has developed a number of plugins that can help with many users issues.
Even with Bbpress scale and amount of plugin developers I still can’t believe there is no way of adding images inline in a BBpress post with the image being loaded into the WordPress media library.
It would be awesome for Matt find another engineer to support bbPress, I know the team is spread pretty thin. v2.6 feels like it may never come. =(