It's getting harder and harder to enable new conformance config bans, mostly due to our very poor type information. Eg, trying to enable #21234 causes 50 warnings:

...
src/utils/xhr-utils.js:160: WARNING - Possible violation: NodeList.p.forEach is not available in IE. Please use iterateCursor in src/dom.js
The type information available for this expression is too loose to ensure conformance.
      init.headers.forEach(entry => {

Now, I know headers isn't a NodeList (and, arguably, Closure should figure that out form the Array.isArray), but because of our poor type information on init, it has no idea what this value is.

There's an internal doc about this: go/unknownprops