Okay. I'll explain it to you. It basically comes down to a server-client architecture. The server (db, server side code) is the backend, that the user never gets to see. The client (basically the browser - js, html and css) is what the user get's to see, not just through the interface, but by selecting "view source".
So PHP and Rails are back-end languages. You never get to see the code and logic that generated the page because it is "at the back". Golang, which the poster mentioned, is also a back-end language. So if the core of your apps logic is at the back-end (server), what you are practicing is back-end dev. If the core is in the front (e.g. SPAs) then it's front-end dev. Sometimes the complexity can be split 50-50 between the back and front. But if your web-application just uses html, css and a sprinkling of JS, then it is back-end driven. Another category is a website, which does not really have any complexity whether at the back or front.
So to correct your statement, PHP and RoR have never, at any point in time been considered front-end tech. If it is not html, css and js (including complile to js languages like ts), then it is not front-end dev.
LOL! Looks like some people are silently getting butt-hurt over a simple challenge asking them to differentiate between front-end and back-end dev. If front-end dev is not your cup of tea, that's ok. Just be honest with yourself about it. Using only html, css and a sprinkling of js is not front-end dev, it is web-design.
If your output is HTML and CSS, you are full-stack dev. There are frontend-heavy full-stack devs ignoring backend with Firebase instead of proper backend. Nobody is telling them they aren't full-stack. So let's stop calling developers backend when they provide backend-heavy solution with a bunch of jQuery plugins or HTMX app.