Requested feature overview description.
The tags of post #4575538 start with:

Artists
? grivies 8

You need to click the "?" to see if the artist has a wiki page.
This one doesn't, like most (but not all) artists with only a few posts.

It would be nice if there were a visual indicator that the artist has no wiki page.
Maybe like this:

Artists
? grivies 8 no_wiki

Why would it be useful?
It would serve as a reminder to create wiki pages for new artists.
And it would save time for people who create wiki pages.
(they wouldn't have to check by clicking the "?")

What part(s) of the site page(s) are affected?
Posts

I think if we were going to do this the simplest thing would be to just change the ? somehow. I'd suggest changing the color to red, because I think redlink == empty wiki page, is already something that most people would understand, but since the tag list is already color coded that probably would not work. adding a strikethrough might, but on a single character it might not be that clear.

I think we should indicate the lack of wiki for every tag type, but I have no idea how.

Adding "no_wiki" at the end might just look like a tag or part of the tag.

To clarify, it's not a wiki page artists should have, but an artist page, which are separate things. Most artists don't need a wiki page since the artist page already covers their alt names and site links (and username if they have an account here linked to their artist tag), and there's in-progress work to have their (C)DNP status automatically show on their artist page too. It's annoying when an artist is given a wiki page that just copies their site links or mentions alternate names, since it's extra work to update when they join a new site or disable an old one, it's ignored for the Name and URL searches in the Artist tab, and the wiki page can't be removed once created.

Maybe a code to color the question mark for wikiless tags something else?

Darker or grey ? maybe? I'd like this for all tags tbh.

Could add an asterisk like "?*" with a tooltip explaining what it means.

Red-linking seems most Wiki-like, but we'd also have to add a CSS tag for customization of that.

I agree that all tag types should have the visual indicator.
It would be equally helpful for non-artist tags, and work identically.

The tooltip is also a good idea.
e621 already uses tooltips in at least one place: "Posted: 2 days ago"

The biggest question is:
What should the visual indicator look like?

1. Change the color

(I don't have color privileges)
Different tag types have different colors: https://e621.net/help/posts
The visual indicator can't be one of these colors.

1.1 Gray

Or any color that's different enough from orange/red/purple/green/blue.

1.2 Lighter/darker shades of the tag colors

Dark orange = no artist page
Bright orange = has artist page
Might be difficult to tell apart.

2. Use another character / string

2.1 Modified "?"

Looks too similar:
? aaa
? bbb
? ccc
? ddd
? eee

Looks weird:

? aaa
? bbb
¿ ccc
? ddd
? eee

2.2 Completely different character

They look different enough.
One is round, and the other is all right angles:

? aaa
? bbb
# ccc
? ddd
? eee

2.3 String

Easier to tell apart:

?  aaa
?  bbb
?* ccc
?  ddd
?  eee

Maybe this is too "loud":

?  aaa
?  bbb
?# ccc
?  ddd
?  eee

I like solution 2.3, because it changes the symbol instead of the colors.
It's simpler, more consistent, and the artist/wiki-less tags are easier to see)

mantikor said:
2. Use another character / string

2.1 Modified "?"

Looks too similar:
? aaa
? bbb
? ccc
? ddd
? eee

Looks weird:

? aaa
? bbb
¿ ccc
? ddd
? eee

2.2 Completely different character

They look different enough.
One is round, and the other is all right angles:

? aaa
? bbb
# ccc
? ddd
? eee

I don't like the character pairs "?*" and "?#" just because they break the nice neat column alignment of question mark, space, and tag:

? aaa
? bbb
?* ccc
? ddd
? eee

But maybe there are page design tricks that can be used to solve that, by adding extra space to the lines without that second character?

---

Another single-character suggestion: what about an interrobang character? IMO it stands out slightly more than a strikethrough "?":

? aaa
? bbb
‽ ccc
? ddd
? eee

On the downside, at small font sizes that could look more like a text rendering glitch than a character with a specific meaning.

chemistrynoisy said:
I don't like the character pairs "?*" and "?#" just because they break the nice neat column alignment of question mark, space, and tag:

? aaa
? bbb
?* ccc
? ddd
? eee

But maybe there are page design tricks that can be used to solve that, by adding extra space to the lines without that second character?

One way to do this with CSS:

HTML

?<span class="hide">*</span> aaa<br>
?<span class="hide">*</span> bbb<br>
?<span class="show">*</span> ccc<br>
?<span class="hide">*</span> ddd<br>
?<span class="hide">*</span> eee<br>

CSS

.show {
    visibility: visible;
}

.hide {
    visibility: hidden;
}

Hidden elements still take up space on the page, so the aaa, bbb, etc. would all be aligned.