Hi there!
To keep costs in check, AMIs older than 90 days will be actively garbage collected.
AMIs older than 90 days were already marked as deprecated as documented on the website and people had to go out of there way and ignore the docs and query the AWS API with IncludeDeprecated=True to query them so this should in theory not affect you unless you ignored all the warning signs we gave you.
Please use the Amazon API as documented here to query the latest AMI when using NixOS on AWS.
We regularly upload new AMIs so that you get kernel updates and important security fixes.