I wish it wasn't, but IE11 is still a major browser, at least too big to just dump completely in my experience. The UK is worse than most countries in that regard though with higher than average IE11 usage.
Indeed, but this is not a problem for this enhancement specifically. IE11 asks for gzipped files, and the server can still send them when the client doesn't support Brotli, based on the Accept-Encoding header.