UD was implemented sometime around 2009-2010ish. Prior to that all UD races spawned in BG with the rest of us (that was some awkward AFK-running).
At first travel was open/free, but difficult. At the time we had very minimal gear, +1/+2 ish, and the path to the UD was long and difficult. (uphill both ways, ya whippersnappers!)
This lasted for some time, actually. At times there was some funny business, but by and large there were some cool things that happened. My bro had his elf captured and perma-killed by some drow. That was a server first, and the RP around it was awesome. The Tolerance is Treason thing was born, those were good times too.
There were some memorable drow amongst the surfacers in those days. I want to say Mithari was one? I could have that wrong, but there were a couple living amongst Beregost if I recall correctly.
Eventually the raids from both sides got a little wonky. Surfacers went and destroyed a temple or something in the UD, the Drow came and burned Beregost down or something like that. It was a real shit-show. At that point travel between the two worlds was stopped completely, save with DM intervention (and the occasional party-transition exploit
When travel was opened back up, the rules (as we have them now) were put into place. That was probably about 2012ish?