Adding Equal Earth Projection in QGIS?

This projection has been added to the 5.2.0. version of PROJ.4.

If you go in QGIS to Help>>About, you'll see that it uses an earlier version of PROJ.4

If you want this projection now, you'll have to wait for the newer version of QGIS (Which would use this updated Proj.4 version), or you can compile QGIS yourself.

In Twitter Tom Patterson posted this tweet and it works:

The latest #QGIS 3.4.0 supports #EqualEarthProjection in two steps:
(1) In Settings -> Custom Projections… create a custom definition with “+proj=eqearth +datum=WGS84 +wktext”
(2) Select the definition in Project -> Properties… -> CRS -> under “User Defined Coordinate System” pic.twitter.com/InWzw3IEiV

— Tom Patterson (@MtnMapper) October 30, 2018

QGIS 3.4 uses PROJ 5.2.0, but it still says +proj=eqearth is invalid. It works in the terminal however, I am able to convert coordinates in Equal Earth projection to WGS84.