Hostility is objective because it's behaviour. I were to punch or insult someone, and the definition of hostility includes those things (it should, right?), then I couldn't bullshit "it's a matter of opinion if I was hostile or not" - it's a fact. However the emotional impact of the punch/insult would depend on the target of that hostility.

I guess sometimes more subjective things can actually be more measurable, counterintuitively.

Sometimes they do. Specially when it's for multiple subjects - human experiences don't overlap completely, but they do overlap a bit. But for that we need to acknowledge that they're subjective.

I wonder if there’s a good example of a space that’s toxic, as measured by the effect on participant’s mental health scores, but only to some participants.

Spaces that target specific groups. Specially vulnerable groups based on sexuality, race, etc.

For example. If I were to crack gay jokes nonstop, most people would at most feel umconfortable... unless they're homo or bisexual, for them there's a heavy (and negative) emotional impact. Same deal with jokes targetting people based on race, gender, etc.