Actually they are still used, e.g. https://graphicdesign.stackexchange.com/questions/tagged/critique and https://buddhism.stackexchange.com/questions/tagged/reference-request – and a community might define special rules for them, like “if it’s a critique question then you need to do this and that”.
The tag isn’t only used to filter or search but to associate a handling rule – what kind of question should supply what kind of information and/or expects what kind of answer.
Though it may be true, as you say, that SO doesn’t use them any more. And from an implementation point of view, they’re simply tags like any other. Except that there may be a special usage/guidance note associated with them (e.g. as illustrated here).