There is no indication in game that these weapons are not usable by thralls, as in they will do 0 dmg (Sword of Crom) or have 0 damage for heavy hits (predator blade).
A lot of the older guides still list the sword of Crom in particular as the BIS weapon for thralls which after you nerfed them to do 0 dmg is obviously not the case.
Now, nerfing something to do 0 dmg in itself is a pretty strange design decision which i don’t agree with (why not just reduce the dmg if you thought it was op? and for predator why not have the heavy dmg be the same as light?), but my main issue is that there should be at least some indication in game that that is the case so players are aware of it. Just add a flag to it to the weapon info panel… It would also be nice to be able to have some control over your thralls attacks (3 options light attacks only, heavy only or balance which would be the same as current), but lacking that, swords like the Predator that have 0 dmg on a specific attack should have that flag/tag in the info panel as well.