You can currently find the IFSC rules on the official website, the current version as well as the 2018 version.
The first point in the introduction of chapter "world championship" states in the 2018 version (paragraph 12.1.1):
a World Championship competition shall be organised every second, even-numbered, year.
...while the same paragraph states in the 2019 version (paragraph 13.1.A):
the IFSC shall arrange for the organisation of the World Championship(s) every second, odd-numbered year
So that's why you saw world championships on two consecutive years (and why there shouldn't be any in 2020).
That answers your first question: this information is still correct.
I could not find information to the "why" part but my guess is that this is to "make room" for the 2020 Olympic games. Climbing will be represented there for the very first time. So the IFSC did not need to make this change earlier.