Hello,
After how many months of staying outside the country in the current pandemic does one lose the green card (becomes invalid)? After the green card becomes invalid in this manner, will IRS be notified or does one have to physically surrender the card to notify IRS and stop paying taxes?

I cannot answer the question about how many months living outside of the U.S. forces you to surrender your green card. This is a question to be posted under Immigration Law, NOT taxes.

That said, once the Green Card IS surrendered, you will file a Residency Termination Statement with your final tax return to inform the IRS of your changed status.

Thank you. Is the Residency Termination Statement downloadable?

When a green card is surrendered, USCIS informs IRS of the same. I'm assuming in this scenario, since IRS has been notified indirectly, taxes don't need to be filed any longer or is a separate residency termination statement still required to be submitted before tax filing is stopped?

The Residency Termination Statement IS still required. I have a boiler-plate copy I can send you for a small fee, but you can find out what is required in IRS Pub 519.