This is supported in AOSP now.

No it isn't. GrapheneOS is directly based on AOSP, so it includes everything AOSP does. GOS devs have made it clear they want you to use gestures, and they only include the navbar at all because it's still in AOSP.

Inverted navbar is in wide a range of other AOSP custom ROMs (including LineageOS, which could be confused with pure AOSP), because those developers chose to incorporate it. But it isn't in AOSP by default.

This doesn’t seem like a mature thing to say just because you want to flip buttons and are annoyed that the privacy and security-focused project didn’t add a feature to flip buttons.

It's an accurate take on their general attitude to polite requests on their github. And the reason the requests are so excessively polite is because people are treading on eggshells due to their diva reputation - albeit, this reputation is somewhat drowned out by their fandom.

It's also not mature to downvote people just because you disagree with them.

And furthermore, the reason I use the navbar is because I want a force close long-press gesture on the back button, so I can more easily close apps and stop their background processes. To me, that's a fairly important feature for privacy and security. That's also likely a big reason Google pushed away from that, and to see GOS devs align themselves with Google in such a manner raises my eyebrow against them, at the very least.