Hi Alisa,
Both a text version and an HTML version are always sent. Your recepients can change their email client settings to load which one they want to see.
If you want to do this on your side, you could create a custom field (let's call it "Text-only") and put that as a field in your subscription center form. Your leads could then set this field to Yes or No.
Then, when sending email blasts, simply make a text only copy of the email you are sending and configure the flow to send the regular version if Text-only is false and the text only version if it is true.