We have decided to release 6.10 first (partly because of all the GDPR BS that calls for additional maintenance features).

b2evo 7 is developed at 80% already and includes a lot of front-end website editing tools.

When release 7 beta would be made available?

To answer the question of what's new...

coming here from this thread http://forums.b2evolution.net/enable-tinymce-for-comments#c111567 asking about possibility to use v7 in its current state for testing.

For production you should not. As long as it’s not an official release, future upgrades will likely fail.

So, the only problem you see is the impossibility to upgrade? I am asking myself which branch should I take to get used to, to develop some new functionality and new skin. There will be no important data there during the dev stage, but once I finish (around the end of the year, no deadlines fixed) I'd like to go prod.
So, should I take v6 branch and customize it up to my needs, or it is better to start with the 7 to avoid doing the same work twice ? I'am asking that because I don't know how much difference there will be between 6 and 7 and did not find any roadmap and progress report.

It makes sense to develop against the 7.0dev branch if you want to take full advantage of the new features. But a v6 skin will also work with v7.

You will just need to install a fresh v7 version before going into production because any dev branches will likely not result in a correct database state.