You can still sign in with the old account even if the email address no longer actually exists. It's just that complications can arise should you actually need to reply to any emails Apple tries to send you if there are issues with the account.
What you should have done back then was not create a new Apple account but instead change the address on the new account. Water under the bridge.
Purchases can be migrated from a secondary to a primary Apple Account to consolidate them. (This feature isn’t available to users in India.) Read about is in this article. —> Migrate purchases from one Apple Account to another Apple Account - Apple Support
However, I would contact Apple about this because I see people posting all the time about those old mac.com accounts causing issues and you may to start using a newer Apple Account. I have also read posts where people claim even Apple can't help them with them so you may not want to transfer purchases into that account.
Refer to this page for Apple Support features ➔ Contact - Official Apple Support
Select from the presented options until you find a solution for your issue, or see if there is a chat or phone call contact method offered lower on the page under “Get more help” (you sometimes have to narrow down the options multiple times before this is shown). If you do not see your issue, keep experimenting with any series of selections until you reach one that offers a chat session or a telephone call and get the representative to redirect you.
