Ran a simple test to check for all the deprecated functions and I noticed, most of the tests are using methods that have been marked as deprecated. Adding a patch to replace them.
Fixed
2.0
Code
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I've taken the good bits of your patch and tested against latest 8.x-2.x dev branch codebase in drupal 9.x with PHP 8.1.
After some more fixing - all is good now. Tests passed.
Automatically closed - issue fixed for 2 weeks with no activity.