Your components has always has been supported by Joomla! 3.5, but the sandbox was not available because for every new version, we need to prepare the sandbox with a lot of hacks inside.
Did you realized that we propose you a Super Admin account ? That's the main thing. It is not easy, and it requires a good knowledge of Joomla! for this challenge.
On top of that, there is only ONE sandbox for everybody.
Cook Self Service is coming with a totally new sandbox engine with manages the sandboxes in a new way.

In future, the sandboxes will always be up to date in a very short time, which is also better for us for obvious security reasons.

Note : By the way, since few months, the sandboxes was not flushed, but now we will flush them again everyday around 0:00 UTC.
So if your project has disappeared, this is normal, you simply need to regenerate your project.
At the moment of the flush, we are making backups of all your datasets in use.

www.j-cook.pro/index.php/docs/versions/225-2-8-7

That's a really good thing to have a Joomla up to date Sandbox.
Great Job!