Today was the deadline for teams to set their rosters prior to the Rule 5 Draft. That Draft is scheduled for December 9, but probably won't happen then due to the impending CBA expiration. But when it is eventually held (assuming that it is) today's deadline will still apply.
The Mets protected four players, which brings their current roster total to 36. They are:
Ronny Mauricio (#3 prospect per MLB.com)
Mark Vientos (#6)
Jose Butto (#13)
Adam Oller (unranked).
Protecting Mauricio and Vientos was a no-brainer, and Butto was expected as well. Oller, a 27 year old late bloomer, was a mild surprise.
As is always the case, some prospects are left unprotected. The most surprising was #10 Carlos Cortes. Of course, the fact that mlb.com ranks him #10 doesn't mean that the Mets do.
In the Rule 5 Draft, the drafting team must keep the player on their active roster all year. So selections are fairly infrequent. We'll see if the Mets lose anyone. We might not see anytime soon though.