What is responsive design?
A site that lays itself out for the screen it is on, from a phone to a large monitor, from one set of pages. Google ranks the mobile version, so that is the one that counts.
A responsive website rearranges itself to suit whatever screen it is shown on. The same site, with the same content and the same address, becomes a single column on a phone and spreads out on a desktop monitor, rather than there being a separate mobile site.
It stopped being optional some time ago. Most people now browse on their phones, and Google indexes and ranks the mobile version of a page. A site that looks polished on the office computer but is awkward on a phone is, as far as Google is concerned, an awkward site.
Being responsive is the minimum, not the finish line. A site can technically fit on a phone and still be hard to use on one: text too small to read, buttons too close together to tap, menus that bury the one thing a mobile visitor came for. Good responsive design is designed for the phone first, and it has to be fast on a mobile connection too, which is what Core Web Vitals measure.
Websites designed to convert, built to be fast, kept improving.
Purpose-driven design, WordPress and Shopify builds, Core Web Vitals in the green, and conversion work every month after launch.