Both Verizon (Engadget's parent company) and T-Mobile were early supporters of Google's push for RCS. Verizon supported the standard on the Pixel 3, so it's especially odd that it won't support it on Pixel 4, at least at launch. In a tweet yesterday, Verizon confirmed that the Pixel 4 will not support RCS when it launches, but the company did not say why.
There's always a chance that Verizon and T-Mobile will support RCS on the Pixel 4 in the future, but neither company has confirmed that.
Engadget has reached out to Google, Verizon and T-Mobile for more information.