Allow to globally set a default for email's format

I think something is misconfigured via your mail handler/server.

What mail handler are you using?

Is your mail server blocking HTML emails.?

i don't intend to deliver HTML formatted emails.

i'm using what i would consider as the contributed email handler that comes with the Webform module. or do you mean the SMTP module that is sending the emails?

to clarify:
when said option to configure a form email handler's format is checked, which is the default, receivers will get a plain text email that contains literal HTML markup.
if it is unchecked, everything is fine plain text.

my intention is to uncheck that option by default for any new webform that is created and gets such handler attached.

You can unset sending HTML via custom code and a hook.