LanguageNegotiationContentEntity::getLanguageSwitchLinks() should use copies of the URL instead of passing the reference directly, so that other modules can modify links individually, like adding attributes. I see that LanguageNegotiationUrl::getLanguageSwitchLinks() does that.
A use case is
Entity translation switcher →
which gets the content language switcher links with \Drupal::languageManager()->getLanguageSwitchLinks(LanguageInterface::TYPE_CONTENT, $url); and then adds some attributes; take a look at http://cgit.drupalcode.org/sandbox-ao2-2860238/tree/entity_translation_s...
Without cloning the URL, an attribute added to one link will be used also in all the other language switcher links.
A patch is on its way.
Needs work
9.5
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Tagging for tests to show how the url can be altered.