How to bypass HTML mail for specific Webform
There are two parts to this answer 1) Mailsystem configuration and 2) Mailsystem template suggestions.

1) Mailsystem configuration

This solved the issue for me and I was perhaps being silly:
Go to /admin/config/system/mailsystem and set Mime Mailer as your default config, well I want that as default.


Then below I had webform config for all webforms, but this was overkill as the settings above already made webforms use Mime Mailer, what I needed to do was add an override for this one email handler:


The specific email handler machine name was email so you can see its key is "email_email".

2) Mailsystem template suggestions

Something I learnt while investigating this is that there are template suggestions added by mimemail.module of the following pattern:

$suggestions[] = "mimemail_messages__" . $variables['key'];


So the above webform email, if using mimemail would use a template named mimemail-messages--email-email.html.twig