i don't not-believe you, but I would be super surprised if the 80% figure is accurate
the more common thing i've seen time and again is someone giving two week notice and then being told to not bother with the last two weeks (and not getting paid for it either). after one of my employers did this to a coworker i liked who was relying on that final paycheck for rent, i quit a couple months later with 0 notice "because it seems like you guys don't do 2 week notice here". I got threatened with a baseless lawsuit for that cheeky stunt and they abused DMCA claims on my consulting website to try to get it taken down
I obviously don't know anything about the company, but it sounds like you dodged a bullet by leaving.
Severance is also only common in large organizations. I've seen few small startups or bootstrapped companies pay severance when letting people go.
Exceptions being PIPs, if you want to characterize it that way; usually a gracious way to push people out without the drama or severance.
I now live in Canada and there are some different requirements re: getting terminated.
I have also worked at two other companies where we informed certain workers of an impending layoff and provided stay bonuses to them. One of them included basically a non-disclosure agreement since the impending layoff was not publicly known yet.
It may not be common knowledge but it definitely happens.
We were told the company was shrinking by almost half once we hit 1.0 on our project, which was still six weeks away. We would get extra severance if we stayed and if we hit the deadline.