I'm a grammarian pedant by nature, I'm in my very late 40's, male, cis-gendered and so on.
That said: the force gendered he or she has been a construction to avoid in technical writing for quite some time. It's not necessarily offensive per se, it's just uninclusive and clumsy.
They as a pronoun is not incredibly new, it's been brewing and in use for some time, and is in fact in pretty broad use: it's most typically the accepted pronoun of either non-gender-binary folks or some who effectively decline-to-state.
The context in which a he or she can be directly offensive is if a given individual or group has informed one of their preferred pronoun, and one is explicitly ignoring said preference, and forcing a he or she upon said individual or group unwanted, whether that be due to one's inner grammarian pedant, one's political views or even just plain cussedness.
To re-iterate my earlier point: in general writing, technical writing etc, it's been long been considered poor practise to gender language unnecessarily.