1/4" brake tubing from the rear brake hose forward to whatever fittings or valving routed fluid to the rear goes back DECADES, in live-axle cars and trucks. 1960s at least, probably earlier than that. I don't know about IRS Corvettes. My '66 Toronado uses 1/4" brake tubing between the combo valve and rear brake hose.
Front brakes tended to get 3/16 tubing because the plumbing was independent LF--RF. Rear brakes were combined down the frame rail, until it split at the axle-end of the brake hose. The combined fluid path used 1/4" tubing, then split to 3/16 where it branched to the individual wheel cylinders.
The 1/4" tubing has NOTHING to do with ABS. Has to do with transmitting 2 wheel cylinders worth of fluid in one tube.
For the record, 1987 Squarebody pickups (and maybe the SUVs) had the same RWAL as the '88 GMT400. Moderately certain that '87 was the first year. Same deal with engines--the TBI small-blocks were essentially identical to the '88s. The 700 transmissions had minor year-to-year updates from '87 to '88, but I think the '87s already had the auxiliary valve bodies.