Steps to reproduce, actual outcome, and expected outcome welcome. See https://www.mediawiki.org/wiki/How_to_report_a_bug

What exactly is expected when entering [https://www.mediawiki.org/ test]?
That at any point in plain text, for example after the character [, an URL could start in the topic title, so the parser should find URLs and turn them into clickable links?
That Flow should support and recognize classic wikitext markup ([protocol://externallink linktext]) in the topic title and interpret it?

Note Flow does support internal links, see the later two links in the title.

Is it really needed ?
I think yes, because it can follow the comportment of sections in the wikicode format, that accept external links.

Note my unanswered questions.

external links will be displayed like the content of the post.

[[test]] is an internal link, [[en:test]] too. Link to another wikis are not considered as external, except if they are written as real external links, like [https://www.mediawiki.org/ test].

Wikisyntax, except for internal links, is prohibited from titles to avoid mess in notifications.

Please also reply to the follwing questions @Bugreporter, thanks!

In T170086#3419002, @Aklapper wrote:

Steps to reproduce, actual outcome, and expected outcome welcome. See https://www.mediawiki.org/wiki/How_to_report_a_bug

What exactly is expected when entering [https://www.mediawiki.org/ test]?
That at any point in plain text, for example after the character [, an URL could start in the topic title, so the parser should find URLs and turn them into clickable links?
That Flow should support and recognize classic wikitext markup ([protocol://externallink linktext]) in the topic title and interpret it?

I think Flow should support external links, like [https://www.example.com/] and display them correctly.

As mentioned in T59153: Allow links in Flow topic titles, "we decided to use the edit summary parser, which does not support external links in titles".

I see this is intentional, but as this is more safe than allowing templates, this may be a nice feature to add.

See also T15205: Ability to use hyperlinks in log summaries and T16892: Linkify plain external links in edit summaries, log comments, block reasons. We can't reasonably support external links in Flow topic titles unless they are also supported in edit summaries. It sounds like there were some spam concerns with those requests.