I'm not sure what authority you have on this subject, but anecdotally speaking as a non-software engineer who would lump himself with "most people," when it comes to white-collar work, I have noticed that all of my colleagues are, at the very least, expected to read email during time off (weekend or PTO) if not respond.

This applies to adjacent industries as well. I semi-regularly get email from firms (in the same time zone as mine) in the 11PM–2AM range.

Again, I'm not purporting to have any more experience than you do, but you can't assert that this is a tech-industry-only phenomenon when I am sure applicable examples other than my own could be found. I wish I could go without, but spending the first billable hour catching up on email is not really acceptable.

In my experience in white-collar and tech jobs since 2019, nobody I've ever worked with has been expected to be reachable after hours. Same goes for my wife, all tech jobs as well. If someone messages me and I'm online, I'll answer, and it's about 50/50 that they say "Oh you didn't need to respond this late, just work on this tomorrow!"

Sample size of six jobs total, three per person.

No it’s not. Realtors respond after hours and weekends. Tradesmen as well. If you want to win business you have to be available when people reach out.