From MLBTR: "The trade deadline is August 31st. ONLY players in the 60-man player pool may be traded. Any player that is traded must go into an acquiring team’s player pool. (As a practical matter, it seems there’s nothing to stop teams from adding prospects to the 60-man player pool specifically in order to trade them. The acquiring team would need to be capable of carrying such players in their own 60-man player pool while still fielding a 26-man active roster of capable big leaguers.)"
The way I read the bold section above is that any player can be traded. They would have to first be added to the 60-man pool to be traded, then carried on the acquiring team's 60-man pool.
I know this won't be popular, but someone has to say it. Maybe the Braves should punt 2020, trade some players of value under short term contracts given a reasonable return - Folty (if he bounces back and has ANY value), Ozuna, Melancon, Inciarte, Greene, Flowers, Markakis, Duvall (maybe even Freeman given the right offer), and begin the window again in 2021. Most of those guys won't be around post 2020 anyway. Inciarte has an option and Freeman is signed through 2021. But overall, those guys only have value for 2020, a season shortened by pandemic, ruined by injury and threatened by cancellation.