This is a misconception which keeps getting repeated. It's easy to use an em-dash on any modern Linux desktop as well (and in a lot of other places).

Though it does still require nominating a key to map to Compose. And is not generally meaningfully documented. So I’d only call it easy for the sorts of people that care enough to find it.

But then, long before I had a Compose key, in my benighted days of using Windows, I figured out such codes as Alt+0151. 0150, 0151, 0153, 0169, 0176… a surprising number of them I still remember after not having typed them in a dozen years.

In electrical engineering I'm still using a few alt codes daily, like 248 (degree sign), 234 (Omega), 230 (mu), and 241 (plus or minus). I'd love to add 0151 to the repertoire, but I don't want people to think I used AI to write stuff....

I've never bothered to read about the compose key, but en/em-dash is accessible (in Debian) with AltGr-(Shift)-Hyphen/Minus too. Copyright (©) is AltGr-Shift-C.

I miss the numeric keypad (gone on laptops) to be able to properly type my last name with its accentuated letter.