Big quote: "The keyboard has a membrane under the keycaps to prevent debris from entering the butterfly mechanism," say two internal Apple documents regarding the 2018 MacBook Pros.
After being slapped with three class-action lawsuits and the introduction of a free repair and refund program, Apple still has not publicly admitted that its butterfly keyboards have a reliability problem. However, Cupertino has privately confirmed that it has taken measures to address the issue in 2018 MacBook Pros.
According to an internal document circulated to Apple Authorized Service Providers and obtained by MacRumors, the company has added a silicone membrane to the keyboard to "prevent debris from entering the butterfly mechanism." The document titled "Butterfly Mechanism Keycap Replacement MacBook Pro (2018)" also advises service providers to take care not to rip the membrane when performing repairs as it would require "a top case replacement."
"Caution: The keyboard has a membrane under the keycaps to prevent debris from entering the butterfly mechanism. Be careful not to tear the membrane. A torn membrane will result in a top case replacement."
Canadian and European versions of Apple's "2018 MacBook Pro Service Readiness Guide" --- also issued to repair providers --- mentions the purpose of the silicone in precisely the same wording. This verbiage is missing for the American version of the same document, but it does refer readers to the aforementioned keycap replacement guide.
The membrane was first spotted by the techs at iFixit when they disassembled a new MacBook Pro just last week. They correctly theorized that the barrier was to keep out the dust and debris. An Apple patent filed back in 2016 for "Ingress Prevention for Keyboards" further confirms the membrane's purpose.
Apple publicly stated that the new 2018 keyboards were redesigned for quieter typing, but it would seem that this is probably just a side effect of the membrane and not its true purpose. Since the lawsuits over the keyboard issues are still pending, it is not a surprise Apple has avoided speaking publicly about it.
While the jury is still out as to the effectiveness of the membrane in preventing stuck keys, those receiving keyboard repairs for their earlier MacBooks should not expect the same type of ingress prevention.
Apple told MacRumors, " Third-generation keyboards will not be offered as replacements under its service program for 2016 and 2017 MacBook Pro second-generation keyboard repairs."
The reason for this is likely because the top case had to be tweaked in the new MacBooks to allow for the membrane, but Apple fell short of confirming this.
Images via iFixit