I'd be surprised if there are any. I can't imagine anyone plunking down hundreds of millions of dollars for a franchise and then deciding to risk their investment by agreeing to the possibility of relegation.
Besides, relegation to where? MLS Next Pro is third division. USL is the second division. Establishing pro/rel under current circumstances would mean getting relegated out of the MLS system and into the USL and promoting USL teams into MLS.
How would that work? MLS is a single entity. The league owns the teams. At the first relegation, MLS would be chucking their own properties out of their own league and replacing them with independent USL clubs which they have no ownership interest in.
There's only one way it could work: MLS would have to completely transform its business structure and convert all the franchises to independent clubs. Theoretically possible, I guess, but I can't imagine the scenario where they would do that. It would put the league in the exact position it's been trying to avoid for the last 30 years.