A number of DNS software and service providers (including ISC) have announced that they will all cease implementing DNS resolver workarounds to accommodate DNS authoritative systems that don’t follow the EDNS protocol. Each software vendor has pledged to roll out this change in some version of their software by the ‘Flag Day.’ Resolver service providers who have indicated their support for DNS Flag day will be making similar changes to their online recursive services on, or soon after February 1 2019.
(source: https://kb.isc.org/docs/dns-flag-day-will-it-affect-you)

For more information please visit : https://dnsflagday.net/