When I say "optional", I mean you can just create a filename.php and start putting lines of code into it, and it'll work fine. But, these days OOP is favored by most and pretty much everyone is using it. So if you are working on code that other people have written, classes are everywhere.
A lot of Wordpress code is non-OOP, though.
It doesn't work for me, on Firefox or Chrome. Chrome reports ERR_HTTP2_PROTOCOL_ERROR