Statistically a disease reaches herd immunity if everyone except 1 / R0 are infected.

For a disease with R0 of 3 (such as covid) this would be 66%. To explain: If two out of three are immune then a disease which usually infects three people will run out of steam.

Has this ever happened in humans? I haven't been able to find an example.

Isn't this why there is a 1918-related flu outbreak every generation and why bubonic plauge used to recurved every generation because there aren't immune?

I think bubonic plague is bacterial, and was solved with antibiotics.

It doesn't matter that it's bacterial to have recurrences.

Also, we never "solved" the plauge, but it died down a lot before antibiotics were even a thing.

What about people born after that point? As immune people die over time from other causes, and as people are born without immunity, the percent of the population with immunity from having had it falls.

Non-vaccine acquired herd immunity is temporary.

I think the idea is that once there is herd immunity, the virus has no way to propagate and ends up disappearing w/o vectors. However, that would require global immunity.

You are right, I haven't thought about that.