I've updated the draft post, mainly to make this sentence simpler and clearer:
TLS-SNI-01 validation in the production environment is reaching end-of-life. It will stop working temporarily on February 13th, 2019, and permanently on March 13th 2019. Any certificates issued before then will continue to work for 90 days after their issuance date.
(It used to try and explain how to check your certificate's NotAfter date from browser developer tools).
I also added a link for staging.