The ts field in the oplog is seconds since epoch and then order of inserts in that second.
Wall time is added in https://jira.mongodb.org/browse/SERVER-27769
Change oplog OpTime format to raw 64bit values
So it is likely a part of that transition as the seconds since epoch is a 32bit integer that will max out in 2038
Sure I don’t see why not. I still use the ts field at the moment.