Yes and No. The way sites were built in 2011 might differ from what you remember.
Returning to WP after nine years – will the classic editor plugin allow me to pretend it’s 2011 as I migrate my site over from squarespace?
There are a lot of sites from 2011 still running with the latest WordPress. Nothing is preventing you from doing what you did in 2011.
I’d like to move my business website to self-hosted WordPress. Previously, around 2011 in my old business, we used PHP/HTML/CSS in theme templates to create customizations for our WordPress site.
I’m not sure what you mean by customisations, but it sounds like you took a pre-existing theme and made some tweaks to it, rather than building something new from a starter theme such as underscores, or relying on page builder plugins.
Suffice to say you can still do that now. The recommended plugins or themes may be different, but it’s the same. Templates are still PHP, styling is still done with CSS, and HTML is still used.
It seems the entire customization paradigm has changed with WordPress & now focuses on plugins.
They’ve gotten popular yes, but the big paradigm change is actually blocks, and it’s a very recent change.
Even then, the big headline policy of WordPress since the very beginning was backwards compatibility.
My preference is make my customizations in raw theme files & to use the classic editor for the blog portion of the website.
The classic editor has very, very little to do with this. Post content is still post content, wether it was written in the classic editor, block editor, or via XMLRPC with live journal, or even on WordPress.com via a Jetpack connection.
Think of it the same way as opening a text file in Microsoft Word. It works the same way as opening a text file in OpenOffice or Google Office, or even Notepad++. Editors are just editors, they may have their quirks but the final product is always just HTML.
You might encounter some people who claim that Gutenberg means a React frontend, but those people are incorrect and grossly misinformed, post content is still HTML in the database and on the frontend. The block editor is just an editor ( albeit capable of much more )
So yes, you can pretend it’s 2011. Even if things have changed, it doesn’t matter, the old way still works.
However, don’t just hack away at a theme, all your changes will get trashed when the theme updates. Create a child theme. This isn’t 2020 advice, it was best practice in 2011 too! And put your functionality in plugins, themes were always intended to just be how your website looks, not how it works. That’s just as true in 2020 as it was in 2011.