It's apparently the primary language for Google Ads. As such, I expect it to be maintained as long as Google continues to sell ads and if they do move away from it you'll have plenty of warning.
https://itunes.apple.com/us/app/google-ads/id1037457231?mt=8
https://play.google.com/store/apps/details?id=com.google.and...
That said, Adwords used to be written in GWT. It's not impossible to migrate, but quite a big job.
Common Lisp was rarely used at Google when I was there, though it's a big company and you do see a little of everything, sometimes due to acquisitions. You might be thinking of ITA software, which is (was?) used for airline reservations.
How’s GWT looking these days?
That's Dart, not Flutter.
And even so, Dart transpiles to JS for the Google Ads. They could switch to plain JS, or Typescript, or even move to a whole new implementation of Google Ads, and pull the plug.
They also used to use GWT, did that hold on?