Correction: I think it was the ability to turn off the in-window "downloaded file bar" pop-up, which could(?) only be dismissed by manually closing it.

There are a lot of different bug reports out there asking for a flag, and apparently the Chrome team added one, then disabled it a few patches latter.

I forget what I was doing at the time (downloading PDFs for a uni course?). But it was a pain in the ass, of dubious utility anyway, and the cavalier UX attitude of "father knows best" left a really bad taste.

https://bugs.chromium.org/p/chromium/issues/detail?id=8966