This WAS around in the early 90's just not a "brand" of WWE.
WWE worked with USWA in Memphis and Smokey Mountain primarily in those days and a LOT of talent came through both.
The Rock, Papa Shango, Men on A Mission, Undertaker, Jeff Jarrett, Brian Christopher were some of the more successful USWA talents but there were many others who failed like Brakus, Tekno Team 2000, Skinner/Steve Keirn, Spellbinder/Phantasio and the original Doink were among the failures, Jerry Jarrett and Jerry Lawler owned it... Jarrett was even gonna take over for Vince if he went to jail so that relationship was as close to WWE as possible.
From the Smokey Mountain side you had Glen Jacobs/Unabomb/Kane, Al Snow, Tom Prichard, Tony Anthony/TL Hopper, Lance Storm, Chris Jericho, Candido and Sunny, The Hardyz and of course their owner Jim Cornette.
The WWF sent their people to both places in return for being able to use theirs and from 1991 onwards you regularly saw guys coming up from USWA and shortly after for SMW... Most of the "new talent" that came into the WWF in that era came through one of those companies until they started picking up WCW cast offs in late 95.
To answer the question though, both fed talents to WCW as well... as they weren't exclusive it meant some talents invariably went to WCW or ECW first. If NXT had been around in the 90's as an exclusive feeder territory, it probably wouldn't have been able to survive. While both Lawler and Cornette had business relationships with Vince, they'd also get commissions from the other companies for talents Vince didn't want... they wouldn't have been able to do that in a WWE owned NXT and as Vince wasn't offering guaranteed money... they wouldn't have been able to get the talent in to begin with. It was only because Lawler and Cornette could make money to cover "bigger names" like a Terry Funk or even The Undertaker in SMW for a few shots...or even see their teams like The Heavenly Bodies go to Summerslam etc. that the younger talents were getting the chance. It was symbiotic, Vince needed new faces and Cornette and USWA needed big names coming in to draw.
NXT is all about WWE...and that wouldn't have flown in the 90's...