This is how the form looks like for buyers with support:
and this is how the form looks like for buyers with expired support:
Do you see any difference? Me neither.
How should buyers know that their support has expired?
I’m really upset with the things you provide for 30% you take 
They can see this info in the right panel:
http://awesomescreenshot.com/0325opz80d
And I guess a buyer who is close to the expiring period will click anyway on the support forum link, instead of leaving a comment…I don’t know…
Two days ago I had a support request from a buyer, in my email was clearly stated that his support has expired, but I wasn’t able to find a nice way of telling him to renew his support package. So I helped him anyway. But I agree with you, they should see this info in the comment form.
No, I have several comments of buyers each day. They use the most easiest way to get feedback.
I think guys @ Envato just haven’t thought about this yet. Would be a good UX addition 
The Same way, Support Tab also should not have the link to the Support Site or Contact Form, Instead, there should be Support Expired Warning.