Why required and optional is removed in Protocol Buffers 3

I’m recently using gRPC with proto3, and I’ve noticed that required and optional has been removed in new syntax.

Would anyone kindly explain why required/optional are removed in proto3? Such kind of constraints just seem necessary to make definition robust.

syntax proto2:

message SearchRequest {
  required string query = 1;
  optional int32 page_number = 2;
  optional int32 result_per_page = 3;
}

syntax proto3:

syntax = "proto3";
message SearchRequest {
  string query = 1;
  int32 page_number = 2;
  int32 result_per_page = 3;
}