There are 96 files with usages of the has method on fields in the AllBlocks class (found by searching the create github for /AllBlocks\.[A-Z_]+\.has\(/).
This is one of the most common cases of mixins that is easiest to fix on the create side. Most of these usage should be able to be replaced with instanceof checks for the specific block classes, this would go a long way to breaking every single create addon again making create addons need less mixins and in general be more easily compatible with Create. In some cases these may potentially be made block tags, although that is probably quite a rare case.

A similar thing should be done to AllItems and isIn. Relevant search: /AllItems\.[A-Z_]+\.isIn\(/, although in this case most of these should be tags rather than an instanceof check (like #7234).