How to get Node Url in RSS view?
It's called Link to Content. 

Path field could also be used in older 8 versions (you could construct the full URL with replacement patterns while overwriting the field output). 

But the Path field is deprecated in 8.5 and you can use Link to Content instead, with Output the URL as text option. 

Read more here, you can also see screenshots: Path field deprecation and backwards compatibility for existing views. 

There is still work being done related to this, and if you get the wrong link in 8.5.1 there is a patch that seems to help: RSS view with fields give wrong URL from path field.