Requested feature overview description.
In the Tags section, allow any tag to be marked as Abstract.
Similarly to how tags have a Type field, a new boolean field Abstract should be added with means to edit this field. Editing this field would be restricted.

Why would it be useful?
On its own it's not very useful. But it's a requirement for some more advanced features.

What part(s) of the site page(s) are affected?
Tags section.

So what's this about? It's about better umbrella tag management. It's about formally describing which tags are umbrella tags, and which aren't. Here's some of the problems and features that one would be able to address afterwards.

Missing a more specific tag problem
There's many different types of umbrella tags, but the ones that we're interested in are the ones representing finite sets. Sets which have a fixed amount of elements and these elements cover all the relevant options. prolapse can either be vaginal_prolapse or anal_prolapse. It's one or the other, and these two options are all the possible options.

One of the problems with such umbrella tags is that we have posts that have an umbrella tag applied, but then they're missing a more specific tag under the umbrella.

For instance, searching vaginal -vaginal_penetration -vaginal_fisting -vaginal_fingering -vaginal_masturbation -vaginal_knotting (there's also all_three_filled, but 6 tag search limit...), we'll notice that there's a ton of posts with vaginal umbrella tag, but they're missing a more specific tag that would describe what type of vaginal interaction is displayed, in this case vaginal_penetration, vaginal_fingering or whatever.

By knowing which specific tags are abstract, we'd be able to detect such tagging errors. If a user adds a spreading tag, then it would be considered an error if they forgot to also add one of the spread_pussy, spread_legs, spread_anus or spread_butt tags. And we'd be able to point out such an error automatically.

In this case, the system could tell the user, "Hey, you entered an abstract tag here. Please use a more specific tag. Here's the 4 options to choose from...".
So such a feature would make tagging

• Easier, because users wouldn't need to remember all the abstract tag sub-options
• Less error-prone, because users wouldn't be able to enter abstract tags alone
• More complete, as it would force the users to always apply more informative tags (tags under the umbrella).

Split normal tags from abstract ones during editing
During tag editing process, there's actually no reason to show abstract tags to the user in the tags textbox. If a post has vaginal_penetration, then there's no reason to include vaginal and penetration in there as those are added automatically. By having less tags in the tags textbox, it makes editing tags easier. Also, if a user removes an abstract tag's subtag, the abstract tag itself should be automatically removed on its own since abstract tags can't exist by themselves.

Instead of having abstract tags in the tags textbox, they should be shown in a separate, uneditable textbox titled "Automatically added". So if you put eye_patch in the normal tags editing textbox, then in the "Automatically added" you would get eyewear.

Populating abstract tags
When creating a post, it would be possible to pre-populate the tagging textbox with some common abstract tags, or populate it on the fly as means to help user get through the tagging checklist more easily.

Yeah, some kind of abstract tag feature is definitely missing from the current tagging system, which leads to all sorts of workarounds and tagging projects where someone has to check that "posts can't be tagged with X without Y or Z". There's also the disambiguation tag familly and everything that aliases to invalid_tag, both of which are just ways to reproduce some of the functionalities a proper abstract tag system would allow for without having to implement that sort of thing.

One could say that this could pose harsh restrictions on things where it's not always clear cut what's what, like tagging specific species of animals when you often can't tell whether a canine-looking character is intended to be a dog, a wolf, a fox or even a canine at all. This could be addressed by having laxer abstraction policies on potentially ambiguous tags, but is an issue worth pointing out anyway.