For reference https://github.com/algo26-matthias/idna-convert/blob/master/UPGRADING.md

Unit tests are failing with error "Class "IntlChar" not found" here: https://github.com/joomla/joomla-cms/blob/5.3-dev/libraries/src/String/PunycodeHelper.php#L44

E.g.:

1) Joomla\Tests\Unit\Libraries\Cms\Mail\MailHelperTest::testIsEmailAddress with data set #1 ('joe@home', true)
Error: Class "IntlChar" not found
C:\projects\joomla-cms\libraries\vendor\algo26-matthias\idna-convert\src\NamePrep\NamePrep.php:69
C:\projects\joomla-cms\libraries\vendor\algo26-matthias\idna-convert\src\NamePrep\NamePrep.php:58
C:\projects\joomla-cms\libraries\vendor\algo26-matthias\idna-convert\src\Punycode\ToPunycode.php:33
C:\projects\joomla-cms\libraries\vendor\algo26-matthias\idna-convert\src\ToIdn.php:55
C:\projects\joomla-cms\libraries\src\String\PunycodeHelper.php:44
C:\projects\joomla-cms\libraries\src\Mail\MailHelper.php:163
C:\projects\joomla-cms\tests\Unit\Libraries\Cms\Mail\MailHelperTest.php:287

This update would require the intl extension of PHP, which probably is installed on a default web server. But it is a requirement change which can be counted as bc break.

I'v opened an issue on upstream algo26-matthias/idna-convert#47.

Once algo26-matthias/idna-convert#49 will be merged and a new release shipped, then this is upgrade safe.

tested that the deprecation notice is resolved by this update.

Unable to follow your test instructions as I dont understand what you mean by " and an invalid one like http:/google.c"

I have tested this item ✅ successfully on f1ff2fd

Updated the instructions, it should be tested with a valid and invalid url.

i didnt spot the single slash in the url

this is needed for php 8.4.5 or Joomla can not be installed if your admin email address is in the form abc@example

I have tested this item ✅ successfully on f1ff2fd

* Tested with current 5.3-dev and PHP 8.4.4 on macOS

• Before applying the PR, after fresh apache restart once the deprecated is shown in Joomla administrator backend

Deprecated: Algo26\IdnaConvert\Punycode\PunycodeInterface::__construct(): Implicitly marking parameter $idnVersion as nullable is deprecated, the explicit nullable type must be used instead in
/Users/hlu/Desktop/no_backup/joomla-cms/53/libraries/vendor/algo26-matthias/idna-convert/src/Punycode/PunycodeInterface.php 
on line 6

• Patch applied with gh pr checkout 45140 and checked in composer* that idna-converser version is updated from ^3.1.1 to ^4.0.4, did composer i and restarted apache, no deprecated error message anymore
• Tested article with valid, invalid and Umlaut-URL; invalid URL is not passing, it is getting Invalid field until it is fixed

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  _{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/45140.}
• Tested contact email-adress with valid address and Umlaut-domain
• ⚠️ Invalid email adresses like heiko@bla are passing without error, same behavier as before the patch

RTC