Removes potentially confusing references to version 3.5.

PiperOrigin-RevId: 845407800

Author: protobuf-github-bot

content/programming-guides/proto2.md

@@ -1165,10 +1165,8 @@ fields that the parser does not recognize. For example, when an old binary
 parses data sent by a new binary with new fields, those new fields become
 unknown fields in the old binary.
 
-Originally, proto3 messages always discarded unknown fields during parsing, but
-in version 3.5 we reintroduced the preservation of unknown fields to match the
-proto2 behavior. In versions 3.5 and later, unknown fields are retained during
-parsing and included in the serialized output.
+Proto2 messages preserve unknown fields and include them during parsing and in
+the serialized output.
 
 ### Retaining Unknown Fields {#retaining}