There was a definite step-change in strictness a few years ago, though. This is evidenced by the sheer number of questions which are marked "this question exists for historical significance, but [don't ask questions like this]" and locked.

That's not a useful metric. The questions that today show up as locked for historical significance were simply closed before the historical significance lock was introduced.

Conversely, the lock was introduced to stop those highly interesting (but ultimately useless) questions from getting deleted. If anything, it's a step in being less strict.

Oh, so that flag was just applied all questions closed before they introduced the historical significance thing?