Per my comment on T289212#8589664 - think this makes sense as a request for Growth team to look into. Speaking as a member of that team, I do think the appropriate action may be to update this task to be about exploring how different prominence of Create account vs Log in can be used to improve growth and retention of newcomers, rather than immediately going to removing it from header.
A secondary aspect may be to improve comms on the benefits of account creation on Special:CreateAccount.

Pinging @KStoller-WMF in case you have thoughts on how Growth may look at how this may fit into focus on the team's work.

In T289212#8589664, @RHo wrote:

In T289212#8587101, @ThurnerRupert wrote:

is there any chance to remove "create account" from the top level and let it be on the login page? there is no obvious help why creating an account is cool. i created one to describe it a little better:
https://phabricator.wikimedia.org/T328836

Hi @ThurnerRupert - thanks for the question. I mentioned in an earlier comment that may have been for a main reason for highlighting "Create account" is that:

In T289212#8576273, @RHo wrote:

... @TheDJ mentions in T289212#8562811 that many sites do have Log in as the main link, but on Wikipedia it is less expected that one can create an account since not having an account doesn't prevent one from reading or editing, whereas on any other sites people typically have to log in to access something, so I believe this was one of the main reasons for making the Create account more primary.

To elaborate, because Create account on Wikipedia is less required than on say an e-commerce site or even online newspaper which requires subscription for full access, one of the ways to surface the many benefits of having an account is to show it as a main prompt for those who don't yet have an account.

What are the benefits, you ask? Besides ability to customise your UI more via Special:Preferences, and access more features like Dark mode, Reading lists (for app users), etc for readers; those who create and account will now also access features specifically created (by the Growth team) to help people start editing on Wikipedia. There are future plans in place to surface these benefits better on the Special:CreateAccount page by the same Growth team. I will cross post this comment on your feature req't task and channel to Growth team for response.

see here the examples from mediawiki, but on wikipedia this is similar. i remember how long it took me to register an account. i thought it is not beneficial. until i found out that this thing publically logs my IP address on an edit - and what people can do with it. apart from the benefits you mention RHo. and - the benfits on one place which is regularly seen - login - would be very good. create account is too rare and creating an account is necessary for login. create account by itself has no benefit. other than blocking your username.

The benefits will probably be specific to the project, so we'd probably want something similar to T325655: Replace SpecialCreateAccountCampaign.php with proper hook for the login page as well.

There was some discussion on merging the login and signup page at T134612: Fix Create Account CTA on Special:Login (and I think in some detail elsewhere, but I could not find that). I suspect it would turn out pretty complex, both pages are generated in a mostly automated fashion.

Some weeks ago we conducted user testing with 5 users to test the new design of the Codex Fullscreen Mobile Dialog T339369. To test this dialog in a real context, we used the "Create account" form to simulate a real scenario. After analyzing the test results, we found some relevant insights about the account creation:

In response to the pre-question about users' familiarity with Wikipedia account functionalities:

1. Some users were unaware of the option to create a Wikipedia account, lacking information about its benefits, such as customizing user preferences.
2. Only one of the five users had a Wikipedia account but couldn't remember the password. Another user had considered creating an account before but just to contribute to Wikipedia. The remaining three users had never contemplated it before.

So, considering the general knowledge about Wikipedia accounts, there may be a need for clearer guidance on the benefits of account creation to encourage users to create accounts.

In case you need to review them, you can find the user testing results in these slides, where I documented the background, challenges, goals, findings, and insights.