Twig's |nl2br filter outputs <br />. The HTML validator discourages this:

Info: Trailing slash on void elements has no effect and interacts badly with unquoted attribute values.

Switching to <br> has been suggested before: #1306
But it wouldn't be a good idea anyway, since a similar PR for <input /> got reverted later, due to issues with some HTML parser: symfony/symfony#47715

So, new suggestion to eventually get rid of those unnecessary slashes: ;-)
PHP's nl2br() function has an argument $use_xhtml to toggle between <br> and <br />. So just add this to Twig and everybody should be happy :-)

I could try to create a PR. For v3 or 4?