Feels relatively significant because 10 years into it, he's on a HOF path and still has another year with an option. Had a down 2019, but I feel like there's gas in the tank especially if he fades to a C/1B/DH hybrid. I think the current list is:

Welington Castillo
Ian Desmond
Felix Hernandez
Mike Leake
Nick Markakis
Hector Noesi
Buster Posey
David Price
Joe Ross
Tyson Ross
Ryan Zimmerman

There are basically two categories here:

Tyson Ross didn't have a job with a team to opt out of. I think American baseball was in the process of opting out on Hector Noesi in general. Welington Castillo was probably looking at taxi squad catcher until/unless somebody got hurt.

Price, Desmond, Posey, Markakis, Hernandez, and Zimmerman are all mid-30s with distinguished marks having made 9 figures and in the twilight of their careers. Mike Leake is similar in age but a cut below in terms of esteem and earnings.

But I believe that Joe Ross so far is the only player under team control and in his 20s, seemingly with more to play for trying to play a "full" season after coming back from injury and having "only" made a few million on the cusp of arbitration.