I assume they had heard of him because he was a really good bounty hunter. Same way Optimus knew Shockwave. The bad guy is infamous.
These guys have been around for millions of years. They've probably run into him during the old war. So they'd know him even though he hadn't been around for the last decade of stuff on earth.
The other tf media has stated lockdown has been hired by the decepticons many times before, and that they are his best customers(or were rather) so prime and ratchet more than likely crossed paths with him before
This. He was definitely the best villain of the series so far. I loved how he just didn't give a shit about anything except his bounty.
I assume he's like the Boba Fett of the Transformers Universe. Everyone has heard of him and know he is not a threat to be taken lightly.
Didn't lockdown ambush prime before he went into hiding? And optimus radioed the other autobots about the human and bounty hunter ambushes as ratchet was responding to his sos before the inevitable happens So this could be how they know off him