I recently upgraded my CSS610 to SwOS Lite 2.18 after just looking at the web gui for an unrelated thing.
Had no idea there was an update available and was thinking, since the web-GUI does a check for a new version and also finds the version and release date, can this info not also become available through its own OID?
That way it can also be monitored, for those who wish, through their respective NMS. Or is this feature already available somehow that I'm not aware about?

I don't think there is anything in the MIB, other than current version. Often you can use HTTP to fetch the current version from a Mikrotik web service, and then have an alert/rule/etc in NMS that compares the result from HTTP with the SNMP version from device.

While Mikrotik has URL to fetch the current version # for RouterOS (viewtopic.php?p=1042832&hilit=NEWEST+url#p1030161) but I'm not sure about URLs for SwOS...

You might want to file it at help.mikrotik.com. They have a "Feature Request" category.

I recently upgraded my CSS610 to SwOS Lite 2.18 after just looking at the web gui for an unrelated thing.
Had no idea there was an update available and was thinking, since the web-GUI does a check for a new version and also finds the version and release date, can this info not also become available through its own OID?

This can't be done for SwOS. SwOS never initiates outgoing connection (necessary for e.g. check for upgrade availability), so it can't present this info in SNMP statuses. When one connects to SwOS device using web browser it's the web browser which checks for uprade availability.