The behavior you described is, apparently, specific to English Wikipedia (and specific to new users), and implemented in the extension ArticleCreationWorkflow. On other projects (or when you're an established user) there is no "New user landing page", instead the editor opens. We'll probably have to handle these cases differently.

(By the way, I didn't know about this extension, but it's actually quite interesting – it essentially adds an "empty state" for articles, just like we're trying to do for talk pages, although the implementation is quite different.)

From the merged task:

Currently the discussion tools empty page message seems to appear on all non-existent talk pages. This is not desirable when no corresponding subject page exist (example image below). The exception to this is for user talk pages where it can be appropriate, but only if the user actually exist. Could the notice be changed to only appear when there is a corresponding subject page or user for user talk?

We also discussed the situation with non-existent users in https://wikimedia.slack.com/archives/GRR5LM0MS/p1628770362079100 (sorry, this link is not publicly accessible).

In T288800#7282553, @matmarex wrote:

Thanks for feedback!

+1 – thank you for filing T288800, @Trialpears.

Two resulting questions for you:

1. Does the table below accurately describe the cases you have in mind as well as the behavior you would expect?
2. Can you please share what you would expect to see in Cases 1 and 3 and why you would expect these pages to look/ function in these ways?

...answers to the above will inform the approach we take in T288319.

Expected behavior

That does accurately describe the cases. I'm honestly not sure how to best approach to handle these cases are, but that "You can use this talk page to start a discussion with non-existent user" isn't it. I think just not displaying anything special from discussiontools would be fine, but perhaps a custom message like "This user does not exist on this wiki. You can check SUL to see if NAME is registered on other wikis.", "The page NAME does not exist. If you wish to create said page you can do so through the Artciel Wizard" or "The page NAME has previously been deleted. If you wish to discuss this issue contact the deleting administrator". Probably too specific for these to be the defaults though and significant localization is probably required.

In T288319#7283809, @Trialpears wrote:

That does accurately describe the cases.

Okay, great. Thank you for confirming this. I've added these cases to the T270323's task description. See: T270323#7299058.

I'm honestly not sure how to best approach to handle these cases are...

Fair enough! We'll file tickets for these cases and mark them as sub-tasks of T270323 once we do.

English Wikipedia has customized the messages to display a big red warning in this case: https://en.wikipedia.org/w/index.php?title=Talk:Dlkfjalskjflaksjdf&action=edit&redlink=1

I think it's clear that we should just make this change, folks keep reporting that it is a problem. We probably should have done it a while ago.

Change 820812 had a related patch set uploaded (by Bartosz Dziewoński; author: Bartosz Dziewoński):

[mediawiki/extensions/DiscussionTools@master] Don't display the empty state if the subject page or user doesn't exist

https://gerrit.wikimedia.org/r/820812

In T288319#8134547, @gerritbot wrote:

Change 820812 had a related patch set uploaded (by Bartosz Dziewoński; author: Bartosz Dziewoński):

[mediawiki/extensions/DiscussionTools@master] Don't display the empty state if the subject page or user doesn't exist

https://gerrit.wikimedia.org/r/820812

@matmarex: of the cases named in T288319#7282807, would it be accurate for me to think the patch you've written above addresses "Case 1" and "Case 3"? And if so, what will people see on the talk page in these cases?

Yes. They will see the old wikitext editor.

In T288319#8138653, @matmarex wrote:

Yes. They will see the old wikitext editor.

Sounds good.

Change 820812 merged by jenkins-bot:

[mediawiki/extensions/DiscussionTools@master] Don't display the empty state if the subject page or user doesn't exist

https://gerrit.wikimedia.org/r/820812

Testing on https://simple.wikipedia.beta.wmflabs.org/w/index.php?title=User_talk:TestEsther looks like: