It might not be clear that the value in the textbox associated with "Signature" could be used as a nick name.
|Open||None||T64559 Redesign Special:Preferences (tracking)|
|Resolved||Volker_E||T180538 Improve Special:Preferences UI/UX|
|Declined||None||T194533 Special:Preferences: rename the field titled "Signature" to "Signature/Nickname"|
I don't think this would be helpful. It isn't used as a "nickname" (whatever that would mean), it's only used in ~~~~ signatures.
The screenshot is from the English Wikipedia, which adds a ton of confusing text there. The MediaWiki message is much clearer: "Treat signature as wikitext (without an automatic link)".
Interesting, I also see the following
note about nickname in meta wiki and mediwiki.org:
Treat as wikitext
If checked, the contents will be treated as wikitext markup and may include links, templates etc. (but not images or external links).
If unchecked, the contents of the box has to be plain text, and will be treated as your nickname linked automatically to your user page.
Comments on talk pages should be signed with "~~~~", which will be converted into your signature and a timestamp.
Which wiki doesn't have a note about the nickname?
MediaWiki's current descriptions seem sufficiently explanatory, for example with current i18n/en.json:
"yournick": "New signature:", "prefs-help-signature": "Comments on talk pages should be signed with \"<nowiki>~~~~</nowiki>\", which will be converted into your signature and a timestamp.",
So the software defaults are not mentioning “nickname” while for example enwiki went its own way of altering the field's meaning to nickname as well. dewiki as 2nd largest language doesn't feature this meaning either.
The issue with current possible confusion in enwiki is better discussed with the community than in this task.