Tiki 27.0Alpha Categories
Sorry all , that was my doing...
Because it was added to 26.x non-optionally and on some sites (with 1000+ categories) looked ugly and seemed unnecessary i made it optional and tried to make it a bit less ugly for 27.x
The new pref is called category_browse_show_categids and is on the category control panel e.g. here tiki-admin.php?page=category&highlight=category_browse_show_categids (near "Show category object count ")
Guess that should be on the release notes somewhere...