I think Reason stopped using RNS extension.

[Oquasec]

It seems like on newer versions of Reason you can only save as .reason not .rns anymore.
Reason 10 is not working with any RNS files and only open .reason files : /

[Marc64]

I can open old legacy .rns files. Why would you want to save as .rns ? the new is .reason.

[PhillipOrdonez]

It seems like on newer versions of Reason you can only save as .reason not .rns anymore.
Reason 10 is not working with any RNS files and only open .reason files : /

Reason 10 opens .rsn files as always, but saves them as a new .reason file.

[Oquasec]

That is weird Reason 8 & up saves as .reason ooooook
Everything with the rns extension is now being called legacy files.

[pushedbutton]

It makes sense, we used to be limited to 3 characters for a file extension, now we're not. .reason is more visible and easier to remember.
if you want to make sure you're searching for all your reason files, including the legacy ones, you can put "*.rsn ; *.reason" in the search bar.
Embrace change, it'll happen anyhow.