I've done a reasonable number of exploration missions without seeing any boffs.
Granted the higher level clusters are a little messed up, like the anomalies only give basic data samples.
Nada so far from what I've seen (Level 37 myself now). I'm guessing this is something else they cut for launch, it's extremely rare, or it's relegated to admiral level special exploration stuff. Of course on most exploration missions you don't even talk to any aliens. I've been ignoring aid missions, but I doubt there tied to that either.