I'm not a lawyer, but are there any potential trademark issues? AFAIK in general you HAVE to change the name to something clearly different. I consider it morally OK, and it's probably fine, but HTTPXYZ is cutting it close. It's too late for a rebrand, but IMO open-source people often ignore this topic a bit too much.
I don't think HTTPX is a registered trademark.
Is httpx trademarked? I couldn't find anything indicating it was.
He would probably win in a legal case, but is he actually going to take it to court? I doubt it. Also I wouldn't be too offended about the name if I were him and for users it's better because it makes the link clearer.
I think if had named it HTTPX2 or HTTPY, that would be much worse because it asserts superiority without earning it. But he didn't.
Don't you need to register and actively defend you trademark for it to apply?