Is PAN or ID card verification necessary to buy Bitcoin in India? There are many Bitcoin exchanges in India but all of them require PAN or others documents to validate identity. However, the Central Bank of India i.e. RBI has not given Bitcoin the status of a valid currency so the exchanges shouldn't ask for PAN Cards or other details but they do.

That's my other question, are these exchanges authorized to ask for PAN card or other documents to verify identity since I am paying through my debit card or bank cccount so additional verification should not be necessary.

Is PAN card or ID card verification mandatory for buying Bitcoin from exchanges in India, even through the RBI hasn't issued any such notification?

The is the only notification by the RBI related to Bitcoin I was able to find find:  https://www.rbi.org.in/scripts/BS_PressReleaseDisplay.aspx?prid=30247
No authority in India expressly authorizes other entities to use PAN or other unique ID cards for customer verification purposes. The entity itself takes such measures since Government issued identities carries trust and uniqueness.
No unique identity of any nature is required for transacting Bitcoins.
Now, as for the Bitcoin exchanges and other trading platforms requiring the use of PAN or other ID cards, these entities have valid reasons to ask for such details, such as:

to avoid legal hassle for the entity when a customer engages in illegal financial activities or anti-national activities  such as tax evasion, money laundering and terrorism. PAN is a must for surveillance, tracking and investigation;
to show itself as a responsible corporate entity engaging with legit customers only and maintaining transparency at their end.

You are most likely to reach this conclusion if you read T&amp;C, Privacy Policy or KYC/AML policy of the entity concerned. For example, CoinSecure has it mentioned here:

No Legal Requirement
Although Coinsecure is not required by the law to follow any Know Your Customer (KYC) policies or implement any Anti Fraud mechanisms, however Coinsecure believes in total transparency and adherence to the law and discourages any of illegal activities while using Coinsecure’s services. In order to discourage and prevent any person from indulging in illegal activities using its services, Coinsecure has made the following KYC and Anti Fraud Policy purely as a measure of prudence and on an entirely voluntary basis.

You must know that there is no standalone data protection law in India and currently, misuse of unique identifiable information is a punisable offense under Section 66C, IT Act 2000 (Amendment 2008), but the punitive punishment has failed to deter many criminals from committing identity fraud, so you must weigh your risks carefully before divulging such critical details.
Side Note: Do read Privacy Policy before sharing any critical information.