Given that it was only in 1945 that Red Skull was transported to Vormir via the Space Stone, who was the keeper of the Soul Stone before him? Thanos is at least a 1000 years old in the MCU, if he knew about the Soul Stone being located at Vormir before 1945, whom would he have found there guarding the Soul Stone? Is there any official source that talks about this at all?
We don't know, no information is given about the Soul Stone before we see it. There are two possibilities: no one was the guardian beforehand or there was someone who got replaced by Red Skull.
The Soul Stone doesn't need a guardian, it's just helpful to give the information about it out. Remember it requires a sacrifice but it doesn't necessarily need someone there telling others this. It's entirely possible that when the Space Stone cast Red Skull out he was the first to ever be cast out and made guardian for the Soul Stone.
Red Skull: A lifetime ago, I, too, sought the stones. I even held one in my hand. But it cast me out, banished me here. Guiding others to a treasure I cannot possess.
Avengers: Infinity War
However, he could have also replaced someone. This doesn't seem likely though, Red Skull is now a spirit-like creature who doesn't seem to age. What would have happened to the previous that they needed replacing? Maybe there's a time limit on it? We don't know. We do know that there was a map to the Soul Stone though so others knew about it. Potentially someone found a previous guardian there and decided it wasn't worth the sacrifice. Or a previous guardian lives at the end and created the map?
Gamora: He can't, Nebula. Because I found the map to the Soul Stone and I burnt it to ash. I burnt it.
Avengers: Infinity War
There's just no information as to if there even was a guardian before Red Skull and if so who they were.
I think it is worth pointing out that the Infinity Stones seem somewhat connected to each other. The Space Stone sent Red Skull to be the guardian for the Soul Stone. Vision's Mind Stone seems to sense when others are nearby as well. As such the Time Stone might have also been around when Red Skull was sent to Vormir and so he could have always been the guardian for the Soul Stone.