This notice showed up in the Admin panel sometime today/yesterday and I can’t find what the issue is. I’ve fully updated everything and followed the hotlink “check your text customizations” leads to /customize/site_texts?outdated=true and I went through every single language option with Only show outdated/invalid selected and nothing shows up in any of them.
I don’t believe this is actually affecting anything but I can’t get rid of the notice
I noticed the same thing was happening here on Meta, and I found an issue where we don’t consider the case where there is an old translation for a translation key that has later been renamed or removed from Discourse core. These cases were incorrectly being marked as having invalid interpolation keys.
I have opened a pull request with a fix for this here.