There are no major Lemmy instances that defederate from Lemmy.ml in particular.
Lemmy.zip comes the closest, not fully defederating but something similar. I forget if programming.dev does similarly as well. (Edit: ah, I forgot - only 2 of the 3 tankie triad instances are thus blocked by default, so here too there are zero major instances that block Lemmy.ml in particular.)
There’s also PieFed.zip by the same excellent admin team as Lemmy.zip. So Lemmy.zip and PieFed.zip are excellent general-purpose recommendations to offer to people. (Edit: despite how they don’t block the triad - but at least PieFed.zip allows individual users to do so, if they wish. And after they figure out how, and that there is a need for such, which nobody it seems will officially recommend to them.)