"The cache invalidation with static was introduced in r9102 with version 2.7. Multisite wasn't in core back then, so something like switch_to_blog() wasn't a concern, but now it breaks if you switch the blog in between calls to clean_term_cache."
This solution is simpler. All unit tests pass. Removes unnecessary tests linked to removed functions.
Props kovshenin.
Fixes#14485, #22526.
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