1 - FastAPI was created before the change was announced.
2 - You assume they knew about it. It's a lot of work to create FOSS, it's very easy to miss those things given the huge amount of information you have to check for compat and security. Hell, I certainly don't assume my main dependency, which is a rock solid language no changing the major version and used by millions is going to break my project. I don't read each line of each changelog of each version upgrade of each of my dependency.
3 - Even if they knew about it, they may never have realized it would have broken something before people told them so.
4 - They may even have noticed, 3 years ago, but forgot about it. It was 3 years ago!
It is surprising that they missed this change, though. Given how core type hints are for their projects and how broadly publicised the changes and developments in type hints were. I'd be surprised if there weren't any issues raised against their projects about those. It feels it'd be harder to miss it than it'd be to stumble upon it.
Also, weird for them to forget that there is a change that will break the foundations of their libraries. You'd think they would have that in the back of their minds and, being stakeholders, even being part of the discussions. Like you said, it's FOSS and, most importantly, python is a community project.
2 - You assume they knew about it. It's a lot of work to create FOSS, it's very easy to miss those things given the huge amount of information you have to check for compat and security. Hell, I certainly don't assume my main dependency, which is a rock solid language no changing the major version and used by millions is going to break my project. I don't read each line of each changelog of each version upgrade of each of my dependency.
3 - Even if they knew about it, they may never have realized it would have broken something before people told them so.
4 - They may even have noticed, 3 years ago, but forgot about it. It was 3 years ago!
It's only human. It's FOSS.