d.o does not autocomplete tags starting with # hash anymore

Confirmed. It would autocomplete such tags on D6.

Issue cannot be reproduced on any dev environment (including clean, untouched ones). When I start typing #Sp, it displays all hash-starting tags. I've checked that on https://sprint12-drupal.redesign.devdrupal.org/project/issues/search, https://sprint1-drupal.redesign.devdrupal.org/project/issues/search and https://sprint14-drupal.redesign.devdrupal.org/project/issues/search
Same for issue edit form.
Staging and live sites still do not display the list when search starts with #, so I believe it's environmental issue

I think the difference is that devwww sites use .htaccess files, while production and staging sites only use Apache configuration, forbidding .htaccess overrides.

Something is un-url-encoding the URL, so the now-literal # is treated as a fragment identifier and dropped.

Since new dev environments are on the way, and will also hard-code Apache configuration, I think the best place to fix this right now is staging.

I confirm that dev without .htaccess and Clean URLs switched off autocompletes #-starting tags correctly

I find that if I type '#' autocomplete returns nothing, but if I delete that '#' and type 'm' instead, autocomplete suggestions starting with # are displayed. Edit: Never mind, the autocomplete sorts items that contain 'm' but start with '#' first.

There are D7 updates to the static rewrites that need to be applied for D7 to function properly. I'm working on applying these now.

This should now be fixed, rewrite rules have been updated for drupal.org

Yep, works! Thanks, basic.