Requested feature overview description.
I remember hearing that the site's design couldn't allow users to follow tags like in some other booru systems. This is a potential solution to that problem.
If you really think about it, a "followed tags" dashboard is just a standard search for an OR-separated list of tags, right? Rather than make a special system for handling followed tags, why not just make lists of tags that users can add to? Just pretend it's a custom dashboard, when in reality it's just a normal search with the list applied on a separate page.
Why would it be useful? There's currently no easy way to check the latest posts on a number of tags at once, short of making a bookmark to the search either in your browser or on your profile. This is a very obtuse workaround, and requires that you update the corresponding link every time you wish to change a search term.
If a link is all that needs to be saved, why make the user do it themselves when it can be handled site-side so much easier?
For me personally, I have so many artists I enjoy, and I want to see when they've posted new things. Currently, there's no way to do that save for compiling the search yourself each and every time, or saving one of each artist's pictures to a set so you have access to their artist tags in one place.
I only use this site on incognito mode, so bookmarks are not an option, and the current workaround of "save the links to your profile bio" I mean really? That's what we're going with as the official solution?
What part(s) of the site page(s) are affected?
My implementation would add a subsection under posts called Dashboard, which links to a search for the default tag set. This would be similar to how Philomena (AKA Derpibooru) handles it.
An optional inclusion would be multiple saved tag sets, similar to public and private image sets.
Regardless, it's just a list of tags that gets separated by OR (or ~) and searched, and presented as its own page.
As for adding tags to the set, I think that could be handled either in the profile or - since it's the closest thing to a "tag" page - in the "?* Wiki link for each tag. Something like "Add to tag set: <dropdown>".
Implementation Details.
Tags would be stored in lists, either of references to that tag's synchronized tag object (if tag names can change) or strings (if they can only get aliased. I'm not familiar.).
When the "dashboard" is selected, the list of tags would be OR-separated (which in current syntax would be prepending the ~ character) and translated to a standard search query.
I'm writing this on mobile right now, but I can provide further clarification and a mockup if desired.
Another thread referencing this: https://e621.net/forum_topics/24373
Updated