Anything above ~1000px is considered difficult to read, yes. For example, the W3C Web Content Accessibility Guidelines require that the site at least provides a mechanism to set the width to 80 characters, or about 1000px at a standard font size: https://www.w3.org/TR/WCAG21/#visual-presentation

You can push it if you want to maximize horizontal space utilization - the site you're on right now, for example, caps reflowing text to about 1200px - but reading is easier when you have to scan over less horizontal distance, and there's literally no reason not to set some max-width.