"The FT8 and MSK144 protocols have been enhanced in a way that is not backward compatible with older program versions. The new protocols become the world-wide standards starting on December 10, 2018, and all users should upgrade to WSJT-X 2.0 by January 1, 2019. After that date, only the new FT8 and MSK144 should be used on the air."
It has also enhancements for WSPR, but for WSPR it is 100% backwards-compatible.

https://physics.princeton.edu/pulsar/k1jt/wsjtx.html

Release notes: https://physics.princeton.edu/pulsar/k1jt/Release_Notes.txt

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