Simply use the pacther from the link below.
https://github.com/RoseTheFlower/SaintsRowTTRNoLetterbox

click on "patcher", download, follow the instructions and that's it! no more letterboxes.
as they stated, it was fully tested on 1920x1080, 2560x1080, and 3840x1080. works perfectly fine on both Epic Games and Steam versions.



- All credits go to RoseTheFlower on Github, just noticed not many people out there know about their patcher and I thought someone should share this somewhere so here we are.
Enjoy.