As long as it's on the same line as the title and easily legible, I really don't see a problem.
And it's not a spot of secondary importance. If it was still in the title, making it longer, the spot where you see the url would have title in it.
Desaturating and down-sizing suggests the information is not important, but in this case is critical to understanding and of equal weight and so should probably have the same visual hierarchy, that's easily achieved by including "Arch Linux" in the title.
You would rate the exact same formatting good or bad based entirely on whether the text next to it is a couple pixels larger. That does not sound like "accessibility".
Whether it's hard to read is an accessibility issue. But that's not your complaint.
I'm not disagreeing with your experience, I'm just disagreeing with part of the way you want to fix it.
Yes, that happens... but I'm not sure how it applies to this specific case? The URL is the same two words that were removed from the title.
I'm not defending HN's title edits in general.