Source: https://screenrant.com/mortal-kombat-4-3-imax-ratio-hbo-max/

This has been one of the many questions fans have flooded the producer with, especially since the Justice League: Zach Snyder Cut was released in a traditional 4:3 format. Todd Gardner affirms to these fans Mortal Kombat will be released as

The theatrical aspect ratio to Mortal Kombat is as follows:

2K, Scope (2.39:1)

Imax is also 2.39:1 with no expanded aspect ratio