A multi-part answer:
This is a very early announcement of intent. We don't have all the details, such as how to deal with Must-Staple, worked out yet. There will be follow-up announcements as timelines and implementation details become firmer.
One possibility would be a transitional period in which we continue to operate OCSP services, but only provide responses for serials that were issued with the Must-Staple extension. This would allow us to greatly reduce the resources spent on OCSP infrastructure, but would not allow us to remove that infrastructure entirely, so it is not ideal from an operational complexity perspective.
Once we've turned off OCSP entirely, there are at least two paths forward for handling CSRs which request the Must-Staple extension: reject the finalize request, or accept the request and simply issue the certificate without that extension (much as we ignore requests for other extensions). Each of these has pros and cons, which we'll be weighing. It's also possible that our approach here will vary by profile. Again, we're happy to take input on what folks think the right path is, and we'll announce firmer details later.
Finally, the privacy risk of OCSP is very real. Without getting into details, CAs in general do get government subpoenas for data of various kinds. And perhaps even more importantly, querying OCSP is a cleartext request which can be passively monitored by entities other than the CA.
We look forward to a world in which revocation is unnecessary because all certificates have lifetimes shorter than an OCSP response has today. But even after we offer 6-day certs, it's going to take a long time to move the entire ecosystem in that direction, and we have to continue to function with both our costs and the whole internet's privacy in mind in the mean time.