Superboy has been back for a couple of years, except that he was in costume only in the future, so the Legion had Superboy, but in Smallville he was just Clark. There was a version of the Mon-El introduction story published that takes place in Smallville, but without costumes (and they fixed the plot hole from the original where the lead box doesn't affect Mon-El). Supergirl has also been back; she joined the Silver Age Legion in her annual from last October (and she's still dead in the Legion's present).

However, DC rebooted their universe yet *again*, just this month. The reboot, this time, has affected pretty much everything *except* the Legion; the Legion got a seventh #1 issue (and a spinoff, Legion Lost) but is still continuing. But since Superman and Supergirl are affected by the reboot, once again, nobody knows what their connection is to the Legion any more--even though their connections have only really been back for a few years. From the vague mentions so far it does seem that at least Superman is still connected to the Legion, but not much has been published yet.