syntax: disallow .example and .alt TLDs

atproto/syntax/handle.go

@@ -31,14 +31,18 @@ func ParseHandle(raw string) (Handle, error) {
 }
 
 // Some top-level domains (TLDs) are disallowed for registration across the atproto ecosystem. The *syntax* is valid, but these should never be considered acceptable handles for account registration or linking.
+//
+// The reserved '.test' TLD is allowed, for testing and development. It is expected that '.test' domain resolution will fail in a real-world network.
 func (h Handle) AllowedTLD() bool {
 	switch h.TLD() {
 	case "local",
 		"arpa",
 		"invalid",
 		"localhost",
 		"internal",
-		"onion":
+		"example",
+		"onion",
+		"alt":
 		return false
 	}
 	return true