I know that BASIC-PLUS definitely used MID/LEFT/RIGHT functions, as one can see on page 5-13 of the manual. It is worth pointing out that these did not exist in the original PDP-11 basic. BASIC-PLUS famously provided the basis for MS BASIC, which is where they invaded our home computers.
It appears SUPER BASIC from Tymshare pre-dates this use, but it seems difficult to imagine how this version might have influenced DEC. I seem to recall Tymshare of this era was all SDS?
Dartmouth BASIC did not use these. The first version to add strings was V7 (circa 68) and it used CHANGE. Wang followed that model with CONVERT.
HP did not use it, nor did the versions of DG's basics on the Nova line which closely followed HP from what I can tell. They all used slicing, and this found its way into Northstar, Atari, Sinclair, etc.
So, does anyone know of a BASIC dialect that used this style of string functions that pre-dates SUPER-BASIC? And anyone familiar enough with these to know how these might have jumped to DEC sometime around 1970/71?