If newer Asus monitors still lack this functionality, it sounds like Asus doesn’t care to include it. :/
To confirm what Bucko said, I’ve bought an Asus VG275Q, recently – which I believe is a 2017 monitor, and TN like OP’s one – and it provides an extensive screen ratio control. Even my much older (as in, 7 or 8 years older) VW246H had a simple 4:3/16:9 toggle, under its own OSD settings menu.
The only thing I can think of, the newer dyspay is labeled as ‘gaming monitor’… Could it be that the more ‘desktop’ or ‘office’ oriented models simply dropped some less relevant feature?
— EDIT —
Aside from the fact OP’s monitor is indeed intended for gaming, according to this video, at around 50 sec it’s possible to see an ‘Aspect Control’ option, under ‘Image’. It’s grayed out, there, but it should be possible to change it from ‘Full’ to ‘4:3’ with sources that provide the correct support.