Once again, no code changes were needed.
Update: I made a serious mistake in migrating from Beta 10 to Beta 13. I thought that I had done so successfully without any code changes needed.
It turns out that I was testing against an older release, not Beta 13. In actuality, the Optimizely module completely breaks the site as soon as it is installed under Beta 13.
After discovering my mistake, I backtracked to Beta 11, where the method checkPlain() has been moved to a different class, that is, from String to SafeMarkup.
When I test this module against a new release of D8, I first carry out manual testing on the intended functionality, then run an automated suite of several tests written to the core Testing module.
When those automated tests all pass, they provide a lot of additional confidence that the module is working correctly.
At the moment, the suite consists only of the ones that I inherited when I started this conversion project. The suite lacks testing of two critical error conditions that must be detected when the user is editing projects.
Now, it's time for me to expand beyond that original set to provide some enhancements by adding new tests. For this module, D8 is a stable enough platform to start looking to new functionality.