[qbasic_forever]

It was removed explicitly for SEO and having a single canonical URL for a resource _for all of time forever_. Right now we use HTML for web pages but who knows what the internet 20, 50, 100, etc. years from now will use--maybe .Super-Awesome-Mega-HTML is all the rage. If over time you are changing your site and its URLs are changing then you're breaking that canonical URL and search indexes, caches, way back machine, etc. all suffer. So the intent is don't make the format of the page (HTML) part of it's canonical URL.

[cube00]

In case anyone hasn't read it, not including the extension is covered in more detail in "Cool URIs don't change" https://www.w3.org/Provider/Style/URI

[hombre_fatal]

The benefits aren’t just SEO. I’d much rather have /about than /about.{htm,html,php,asp,etc}. I don’t see how the latter is preferable for routing to pages.

[paraknight]

I second this. That's what mime-type headers are for. The URL should locate a resource, as the name suggests, not necessarily convey metadata about what the resource is.