As far as I can tell, the only way to hope for this to change is to boycott Spotify as a "podcast" platform. They want to lock their platform down, and only allow us to listen to their narrative and journalistic audio content, then we have to stop using it, and refuse to listen to their content. I don't think it should be considered podcasts if it isn't open for everyone to listen to. I think the only thing that will convince them to open it up and allow us to add by URL is their revenue being affected.
But frankly, this is probably not possible. They bought Gimlet media, and although existing podcasts will not be made exclusive, all the new ones will. They have too many users who just don't care about this. Only we happy few who go just a little bit deeper than the mainstream podcast audience care about doing things like adding unique podcast URLs. Look, I don't even have a problem with them offering exclusive content, I want podcasters to get paid, and I know it's challenging. That's why there were networks like Earwolf and Gimlet. What I have a problem with is this issue; that we can't add podcast content to it (it's not a podcast if it's not available on other platforms, it's narrative or journalistic audio). I understand that Spotify wants to be the best audio content provider out there; they want to provide all kinds of audio content, but I don't want to be locked into their platform.
Also, in my opinion the UI is terrible. Not sure if they've changed it in the past year, but it's astonishing to me that a single developer made Overcast, which is a great UI, great functionality, and a multi-million dollar company like Spotify has a **bleep** UI. I advise everyone to stop listing to non-music content on Spotify and use Overcast. Maybe if enough people stop, they will pay attention and pay some people to develop a good UI and add some critical features.
Basic feature that definitely should be added. In my case, I support various podcast creators on Patreon. As a reward, I have access to separate rss feeds that contains weekly bonus episodes. I have to use other podcast apps to access these because Spotify doesn't support it...
One example of the poor user experience that results from not having this feature: if you search Google for "npr tiny desk" you get to https://www.npr.org/podcasts/510306/tiny-desk-concerts-audio. This page has buttons to subscribe via Apple, Google, etc. but not Spotify. However, there is a standard RSS Link which could be used with Spotify, if Spotify supported this.
Without this feature, users are likely to subscribe to some of the fake (fraudulent?) NPR Tiny Desk Playlists that come up in Spotify search such as: