str_contains(): Passing null to parameter #1 ($haystack) of type string is deprecated in docroot/core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php line 114

I have done a local patch to replace line 144 by:

if (!$this->request->server->has('WINDIR') && !str_contains($this->request->server->has('SERVER_SOFTWARE') ? $this->request->server->get('SERVER_SOFTWARE') : '', 'Win32')) {

That fixes the issue for now.

Posted feedback on MR.

Just for clarity's sake as I don't see this mentioned anywhere, but this happens, for example on CLI (e.g. Cronjobs etc) where $request->server->get('SERVER_SOFTWARE') obviously isn't available and returns "null".