Several players have already elected to opt out of the 2020 MLB season. Those players that remain under team control for future seasons, whether or not by guaranteed contracts, will simply forgo salary and service this year (unless they are considered in an at-risk class) and then step back into their remaining contract situation.
We will keep track of those players that have opted out in this post and update it if any more join the list (links to post on decision):
Welington Castillo, C, Nationals
Francisco Liriano, RP, Free Agent
Nick Markakis, OF, Braves (since opted back in)
Tyson Ross, SP, Giants (released before decision was reported)