Won't this functionality be obsolete once we build the collapsible sidebar for modern Vector? T247032

I doubt .skin-legacy is present since MediaWiki 1.33.

It's not no, the new Vector is in 1.35. To support pre 1.33 however you have several options:

1. add the class via hook using OutputPage::addBodyClasses based on version number to older MediaWiki versions < 1.35
2. Use the selector :not(.skin-legacy) .tux-navitoggle { display: none; }
3. We'll be providing an mw.config value soon, which you'd be able to check and update the code in resources/js/ext.translate.navitoggle.js to not run when the skin version is 2

I'm sure there's more ways to do this, but these are just that came to mind.

To me option (3) seem best, since it is direct feature detection. Can you ping this task when that is available?

Given this seems to replicate the functionality of #mw-sidebar-button may I suggest you check the existence of that (particularly because it breaks the side bar button there) ?
https://meta.wikimedia.beta.wmflabs.org/wiki/Special:Translate?useskin=vector&group=%21additions&language=en&filter=%21translated&action=translate

Change 606248 had a related patch set uploaded (by Nikerabbit; owner: Nikerabbit):
[mediawiki/extensions/Translate@master] Skip navitoggle on new Vector

https://gerrit.wikimedia.org/r/606248

Change 606248 merged by jenkins-bot:
[mediawiki/extensions/Translate@master] Skip navitoggle on new Vector

https://gerrit.wikimedia.org/r/606248