I think the underlying issue is not "that it goes too far".

I don't believe anyone would oppose "going this far" in terms of reducing or abolishing actual slavery.

I think what pisses people off is the utter frivolousness of it all. We all know that this is actually not doing anything about actual slavery.

Replacing the word slave in Python or software in general will do actually nothing for actual slaves in the world nor will it change the minds of actual slavers and actual slave owners.

So whilst people probably do not care that much about which specific word is used, they get pissed off by the frivolous sanctimony displayed by the proponents of the change.