Comment 7 for bug 1729536

@Eric for the temporary downgrade you can force versions in apt (but this will stop upgrades, so ensure to unlock it later on with the same command without a version qualifier). Also you can usually only go to versions that are the latest in one of the pockets (release/updates/...).
So atm that would be 5.7.11-0ubuntu6 which is way too old I'd think.
$ apt install mysql-server-5.7=5.7.11-0ubuntu6
You also have to do the same with each dependency (to downgrade all of them)

If you have nodes that did not yet upgrade you can hold the current version via:
$ apt-mark hold <package-name>

Finally there are all versions still available, just not through apt (which is mostly meant to keep you up to date, not to go back). So for each version there is a launchpad page e.g. for the last mysql version there is [1].
You can go to your architecture on the right and then fetch the .deb files as needed.
Once you have all .debs you want to downgrade you can do so with:
$ dpkg -i <list of .debs>

Of course this isn't really recommended as this was a security update, but as a workaround you might consider it.
It might help if you state on the upstream bug that it was the stable update 5.7.19->5.7.20 that (seems to have) triggered this.

[1]: https://launchpad.net/ubuntu/+source/mysql-5.7/5.7.19-0ubuntu0.16.04.1