I had the same problem last week when I first uploaded my add on ‘anti-social’.
Wanting ‘anti-social’ as a slug, obviously, I found - perhaps not surprisingly - that AMO told me that the slug ‘anti-social’ was taken, and asked me for another. I selected ‘anti-social-blocker’ and went on my way.
So now I have this page:
Out of curiosity, I manually munged the URL and visited this page:
There’s nothing there.
I agree with @jorgev that perhaps, in this case, as well as @ownmaster’s, something used to be there which has been temporarily disabled, or that some abandoned add-on has taken it and had the page removed (but the slug not returned to availability).
But I do wonder whether maybe it is worth looking again at the slug generation / submit part of the AMO site to see if it is sometimes not just generating a new slug but registering it as ‘used’ overkeenly, resulting in unlucky people like myself and @ownmaster having to alter their preferred slug to something else at the second stage of the submission process. It’s the kind of subtle bug that few would bother to report: you just think ‘oh, my cool name has been used before, well of course it has, drat, now what do I do’, and quickly think of a reasonable workaround.
But that’s two of us now saying this has happened recently.
Anyone else recently submitted a new thing and had to change the ‘in use’ slug?
Better yet, anyone else recently submitted a thing and not had to change the ‘in use’ slug?