Title says it. If you kill a wandering trader, or the llama falls in a pit, or the trader/a llama walks into the nether portal, it drops a lead This is of course a rare item, and I got four of them because of this. I think this removes the function of going to swamps and kill slimes, because you don’t need to do that anymore if you want a lead.
Slime is a relevant resource for more important items than leads. More technically oriented people will want to look for slime anyway. They see swamps less of a source for slime (because it’s a bad source, slime chunks are way better), and more of a potential location of a witch hut.