That link had another discussion in it which gave me some scary responses. Apparently, the answer here Over 100 Form submissions on free tier? mentions that Netlify won’t throw any error, rather deactivate my account if my limit is reached and I don’t add a card within a grace period. Probably not a risk I’m willing to take. I’d just use a different service to receive form submissions.
Thanks for pointing to the link.
EDIT: If Netify takes feedback from the forum, it would be a nice feature if Netlify will reject the AJAX request or something, so, at least we can show an error on the website.