Thing about linux is they don’t remove anything until its an issue for other technologies.
I’m gunna say your best bet is to use a newer kernel. If you’re happy with 1400X1080, then it doesn’t matter. I don’t really know what else to say, though. Someone smarter than me will know what to do.
Have you tried just using the in kernel driver for your video card? The linux kernel now has the AMD drivers in it I believe since 5.0. I’m not sure though. You shouldn’t have to install the proprietary driver unless you have other issues.
I’m sorry that I didn’t updated the topic till now.
Well… now I have the latest kernel 5.2.2 and installed Mesa drivers and nothing have changed regarding my issue…
I think that my options to solve this matter are pretty limited to:
Maybe someone, in the future, would figure it out or new versions of Adrenalin/Mesa would fix this.
I thank you guys for your interest in the matter and the time spent here! 
Did you try the default open source drivers in the kernel?
Like I said in the beginning, I’m new to Linux. 
Umm…I did not tried the default open source drivers from the kernel. How I can do that?
just install the os
I’ve done it 2 times till now. A clean install of the OS and setting up the custom resolution and it didn’t work.
Pop OS has built-in drivers in the kernel and the kernel is 5.0.0.21 which is pretty recent and I think the issue of scaling was even here, in 5.0.0.21.
I will try to change my RX 480 for a GTX 1060 and see if I can solve the matter this way.
IIRC I read it somewhere here on the forums that for the time being, the RX 480 has issues with some of the past, and current drivers in the Kernel. So I’m going to assume that includes the proprietary driver.
Edit: Please correct me if I am wrong.
Indeed it might have been because of that. In the past, on Windows I used to install just the 18.8.2 drivers cause the newer ones had some issue where I couldn’t bring up front the Radeon panel and I couldn’t control anymore the video card with MSI Afterburner. And, of course, my problem written down in this topic was included with those drivers.
That is a 1920x1080 screen, why are you running 1440p on it.
Well…it’s not 1440p because I want to have 1440x1080(not 2560x1440). I think you misunderstood the resolutions a little bit.
Yeah okay I did miss that but why not run it at full resolution then? What am I missing.