The ~2016-designed [0] third-generation butterfly keyboard (MacBook Pro 2018) features a silicone membrane [1] designed to «prevent and/or alleviate contaminant ingress» [0] thereby softening or preventing dust-caused key failure.

[0] http://pdfaiw.uspto.gov/.aiw?docid=20180068808

[1] https://ifixit.org/blog/10279/apple-macbook-keyboard-cover-u...