Dear Nilesh Ji,
Post merger, the transferor company simply cease to exist. What will you do by keeping the registration? That registration is of no use because the legal entity is not into existence.
S. 29(1)(a) the business has been discontinued, transferred fully for any reason including death of the proprietor, amalgamated with other legal entity, demerged or otherwise disposed of;
The above highlighted business:- The interpretation of word 'business' is not restricted to Pvt. or Public Ltd. or something else. Proprietorship is also well within the ambit of business. Company is generally a business organization and Proprietorship is individual business. I think even proprietorship is covered well within 29(1)(a).
Query: What if you provide renting business using Proprietorship GST Registration?
You may argue cancellation needed not needed before the department. But, what about cases when you issue invoice, collect taxes, file return using Proprietorship GST Registration. Can a non-existence company do all this? Anyway, if I am department, will pull your hard under the ambit of operation of law (registration became invalid), and still you acted like a registered person, hence SCN.
Even how logical it sounds to keep registration of dissolved company and thereafter using that dissolved company to provide taxable supply after six months.
*The view is to the extent of understanding of merger as dissolution of legal entity.