The Pixelmon wiki page of Repels states that using repels will prevent any wild Pokemon from spawning while it is active.
This varies from the actual Pokemon games in a slight but very crucial manner. Using repels in the official games only prevents wild Pokemon at a lower level than the first Pokemon in the party from spawning. Usually, the first party member would be at a level higher than all of the wild Pokemon on the route meaning from a practical standpoint, it's as if all the wild Pokemon in the route don't spawn at all.
However, using the repel trick, we can take advantage of how repels are supposed to work and make only the highest level wild Pokemon on the route spawn, by changing what the first Pokemon in our party is to be one of an appropriate level.
This change would make hunting down repels more useful as they can be used to hunt for very rare high level Pokemon (such as the Ultra Beasts, one of the things the repel trick proves to be useful at in the gen 7 games.) In my single player world, for example, I'm near 100% dex completion and Celesteela simply refuses to spawn. I've camped Ultra Savannas for months now and I wish I could take use of a feature present in the actual games.
Alternatively, it could be possible that the repel trick already exists in Pixelmon but isn't documented anywhere, and I've just been really unlucky not having a high level Pokemon spawn yet. Either way, I'd love to have confirmation on just how the repels in Pixelmon are supposed to work by someone in the know.
Sources I consulted:
https://bulbapedia.bulbagarden.net/wiki ... epel_trick