I entered my feed slug as “myslug” so that the subscribe URL is mywebsite.com/feed/myslug. But the url mywebsite.com/myslug also works as a subscribe URL. But this is a problem because I want to have a page at that url. How do I prevent that URL (without /feed/) from serving the feed?
Huh, indeed that is the case, I did not know that 
It’s something WordPress does, I don’t think there is anything you can do about it.
Sheesh, really? I thought this was a pretty basic use case. 
Many people use mp3 or m4a etc. as their feed slug and there is probably no incentive to create a page with the same slug
I suggest you change the feed slug if you haven’t published the feed yet since that URL is barely seen by listeners anyway.
Fair enough. I suppose if I want a more “friendly” URL, I can add a rewrite rule or a redirect.