A few days ago the USLPA and the USL came to an agreement to restart the 2020 USL Championship and USL League One season, but according to a story in the Washington Post, some teams might not participate once the USL season begins again.
According to Steven Goff, writer for the Washington Post, three people familiar with the situation are saying that some MLS organizations may sit out their USL clubs for the 2020 season in an effort to save money as the clubs has become financially strained.
The Washington Post article also stated that a MLS team would have to negotiate such a move with the USL and might not be allowed to return in 2021. Neither the MLS nor the USL wanted to comment.

Currently there are nine USL Championship clubs and five in USL One that are owned by a MLS team, four USL Championship and one from USL One are affiliated with a MLS club.
The USL Championship is planning to kick off the 2020 season again in a regional format with clubs playing at their home venues without fans. The planned start date is July 11th. USL League One is looking to start the season on July 18th.
Note: Original story by Steven Goff appeared in the Washington Post on June 12th, 2017