Using "he" as the default singular 3rd-person pronoun goes back centuries, not decades. It was sexist to varying degrees, but never all that close to truly gender-neutral since modern English itself goes back only so far as times that have been pretty close to maximally sexist. But you can see it plainly in the King James Version of the Bible for example. You won't find any singular "they" there in the sort of places where its use today is novel. There are of course plenty of places where its use is not novel at all.

The late 20th-century innovation was to write out "he or she" in the many places where it seemed necessary, because we didn't have any single word that would fit. Using "they" to refer to "someone", "anyone", or other referents like that was perfectly normal as it has always been. The examples you provide are most naturally thought of in that way and would not spook the old people today. Using "they" to refer to "a student" or some other specified but unnamed individual would on the other hand often seem wrong to people just 30 years ago, but one might sometimes get away with it depending on the audience and the grammatical circumstances. Using "they" to refer to "Jason" or other such specifically known and named people in general was not done, never had been done except perhaps by the occasional poet from centuries past, and everyone would just wonder who you were talking about even if they'd been named earlier in the same sentence. Calling Jason a "she" would also seem odd, but not nearly as odd as calling them a "they"; and if what I've read is at all representative then roughly similar logic would've usually applied in centuries going back to fairly near the start of modern English.

As may still come in handy on occasion, that short-lived move towards using the hideously awkward phrase "he or she" gave many of us plenty of practice in simply avoiding all phrases that call for a gender-neutral 3rd-person pronoun. Whatever else might be said about it, being able to use "they" is certainly an improvement over that situation.