About a decade ago it was a pretty commonplace thing to happen.

HTML 4/4.1 was kind of messy, and could have rendering issues. So going with an (x)HTML validator was a common thing, as well as a marketable value proposal to clients.

HTML 5 had much "saner" implementations, so validators fell by the wayside as they weren't as necessary for compatibility.

A good text editor will validated against the DTD as you type. And after you publish, you can use https://validator.w3.org/

The Firefox source viewer (not the developer tools DOM viewer) does validation. It will highlight bad tags in red and if you hover over them it shows the error.

I'm pretty sure Moz has HTML validator built into its SEO tool, so it may be more common than you think solely because of that. We validate HTML at my company—If we don't we'll hear about it next time our boss runs an SEO check,