Martin Splitt from Google responded to a question from an SEO around if Googlebot support fetch (or XMLHttpRequest) POST-requests? Martin said he had to test it and he did and he said Google does indeed support simple POST requests based on his test.

He even showed the example code and results from the Google testing tools.

He posted this on Twitter where you can access the code example and see screen shots of Google rendering the page and loading it.

Here is that tweet:

I am not sure if this is new, I think we saw examples years ago of Google doing similar things?

Forum discussion at Twitter.