ACL for D8 and D9 — BETA1

Thanks to EclipseGc we can finally go to beta! Getting the testbot to accept our tests proved to be amazingly difficult...

At the time of writing, ALPHA2 has about 3500 known installations and no open bug report.

BETA1 should now run on D9 as well as on D8.

I'm releasing BETA1 because getting the tests green is a major milestone. The other changes are related to D9 compatibility.
Please do update to BETA1!

This issue is intended to record testing progress.

Please test this version and provide feedback, both good and bad:

We need specific testing in the following areas:

1. Running under D8 (user's point of view)
2. Serving D8 client modules (developer's point of view)
3. Running under D9 (user's point of view)
4. Serving D9 client modules (developer's point of view)

Unfortunately, Devel Node Access for D8 is not functional yet ( #2643136: Plan to port DNA to D8 → ).