The FileMaker 12 help makes a reference to this (see here).
That said, it is unclear if the feature was introduced in FMP12 or started to be documented at that specific location from FileMaker 12.
Basically, this was introduced on or after sub-summary parts would display in browse mode (prior to that, they displayed only in preview more). That feature, made it so FileMaker's default sort behavior would be to keep records sorted even if you alter a record's data that is targeted by the sort order (if a first name "Sam" is renamed "ZZZSam" and your records are sorted by first name, FileMaker will push the record where it belongs in the sort order.)
Prior to all this, if you were to edit a record's data that was involved in the sort order, the record would not budge and the sort order would become "semi-sorted" (what you get today if you uncheck that checkbox).
I believe the change to the sort behavior was initially called "persistent sorting" or "automatic sorting". I also think this change was present in FileMaker 10, but do not know if it was in the product prior to that.
HTH
P.S. Thanks for the walk down memory lane!