But I like to write paths like /usr/bin/ in my text and don’t want to worry about backtick code quoting them every time
Only if the intent is for the URL to be copy/pasted, otherwise you enclose the URL in <url> or [text](url) to make it clickable.
Since when?
I will say that forward slashes are still more common in regular english text even among non-programmers than underscores. For example, listing options a/b/c.
And you know, URLs.