For visitors, they remain as useful as ever. If they’re useful for visitors, that helps us too. Discarded the idea? I’d encourage a rethink.
Once upon a time, it was normal to ‘tag’ every web page for our visitors. Whereas ‘categories’ showed the hierarchy of our website, perhaps more related to the type of content, tags led us to every page about a specific subject, whatever the content type.
Then social media took up the idea (as ‘hashtags’), and it could be argued that the tags on our websites started to look a bit like spam, even though that wasn’t the case. That’s the main reason I can think of for their reduced use anyway. For visitors, they remain as useful as ever, and if they’re useful for visitors, that helps us too.
So for those of us who’ve discarded the idea, I’d encourage a rethink. Can they help with SEO too? Probably not much, as the search engines should be able to identify what pages mention any given keyword – it’s what they do. However, if we have a list of critical tags/keywords on the home page, linking to an index of pages with those tags, it brings the page just two clicks away, which can be useful.