What happens if a Route 53 name server receives a query without EDNS0 information?

Assuming the query is otherwise valid and directed to the correct server, it would respond. EDNS50 is an optional extension to the DNS protocol; it's absolutely not required and a good number of clients do not in fact support it at all.

Hello!

EDNS0 is an extension that will let DNS clients include extra details in queries, like maximum packet size and other options. It is a truncated version of the client's IP address" so include extra details in queries, like a truncated version of the client's IP address, maximum packet size and other options.

When a DNS query is sent to a Route 53 name server without EDNS0 (Extension Mechanisms for DNS version 0) information, the server will treat it just like a regular DNS query. EDNS0 is an extension that will let DNS clients include extra details in queries, like maximum packet size and other options.

If there's no EDNS0 information in the query, the Route 53 name server responds using the standard DNS protocol. It will not use any special EDNS0 features. Rather, it will follow the regular DNS resolution process and give the DNS response based on zone configurations and also records.

Reference - https://docs.aws.amazon.com/Route53/latest/DeveloperGuide/routing-policy-edns0.html