Re: "Airpods not charged for months due to lost charging case"
"Are these still okay to use if I will just buy a new charging case, or should I buy a new one completely?
I'm worried that my money will go to waste if the battery of the AirPods has been degraded."
Sorry I do not know whether or not the AirPods battery will be ok long-term or for further use, yet your request deserves some response. It may depend upon other factors too, such as the AirPods charging pattern when they were in use
Apple has an article online that may be useful: Charge your AirPods and learn about battery life - Apple Support
Eg: article says: "If you think that your battery has an issue, learn about AirPods service and repair."
(You can also get an estimate for a replacement case, (or airpods))
You may want to reach out and discuss the situation with the AirPods Support team:
"Find all the topics, resources, and contact options you need for your AirPods...."
Their website: AirPods Support - Official Apple Support: