If you open such a file, you can set the syntax definition to R yourself in the file properties. Textastic will remember your preference and use the syntax definition you selected for all files with the same extension.

You can invoke it by tapping on the console button in the toolbar on the preview screen. Here is a screenshot:

It still works for me in Textastic 6.3. I haven't removed it.

Textastic 6.3 adds a new JavaScript console that can show errors in your JavaScript code and allows you to log custom messages using the JavaScript Console object (for example by calling the console.log() method).

Textastic 6.3 adds a new JavaScript console that can show errors in your JavaScript code and allows you to log custom messages using the JavaScript Console object (for example by calling the console.log() method).

This has been added in Textastic 6.3!

This sounds like a nice idea, but it's probably out of the scope of Textastic because it is so specific. Maybe it would be a good idea for a new app.