The word "threshold" is commonly understood in computer science (and probably throughout engineering). I suspect it has a formal definition in technical dictionaries that has no relation and probably no reference to its etymology. In that field, I don't think it's a metaphor - any more than "semaphore," "key," or "web" - at least not in the minds of those who use it in this technical sense.

The audience of my writing is generally technical - engineers mostly, hard scientists, but occasionally a non-technical person who is otherwise surrounded by technical people. My guess is that even this person would understand the word threshold in the context I have described. I don't know for sure, but I strongly suspect that none of them (or only a very few) would be aware of the etymology, let alone be confused by the use of the term.

Of course I could (and often do) use the terms "upper limit" and "lower limit." But it makes otherwise dry text less monotonous, if I mix up the vocabulary a little. Also there are some few cases when I need to refer collectively to upper and lower limits.