Yes and no. Mostly no, I think.

In a lot of cases (most?) even plain-text email is rendered in proportional fonts which don’t work for ASCII art.

The default for all mobile email clients, Gmail, Outlook, and Apple Mail is now to render plain-text in proportional. Those who get it monospaced have chosen to do that. It’s also made worse by Outlook’s insistence on removing ‘extra’ linebreaks by default. AFAIK, there’s no way to switch off that behaviour except email-by-email, and you can’t know if your recipient has it or not.