Thesaurus entries have always been able to be updated "in real time" (going back to MDEX 5.x?, maybe earlier?). It's not really "real-time" but it is fast and the index does not need to be taken down.
It's not really a "partial update" as partial updates update records. This is a configuration update (which includes thesaurus updates) and it modifies configuration (not records) which is maybe where the confusion arises.
For more info, check here: