Is there any unsaid rule that Magadh University candidates cannot join TCS?

Yes it is true because TCS blacklisted Magadh university

Not sure dude but Magadha bank is surely black listed after lucky Bhaskar movie

My friend have been denied to give interview in a recent walk in saying he is not eligible to appear as his university is Magadh University

There are blacklisted institute. But magadh university is one of them or not no body will tell you.

Hiring team does not reveal such information

There is no such rule. Magadh University candidates can join TCS just like any other university graduates.