> Arguably it is. It was sent as plain text and received as plain text. The fact that the recipient's software goes through and interprets that plain text and does something when it detects an URL in it doesn't change that. If the recipient were to use some other software that doesn't do that, they'd see... Plain text. Because that's what it is.

Exactly like .... the subset of HTML we see in emails?

I have not yet gotten an HTML email that, when displayed in plain-text, was unreadable. Neither, I suspect, have you.