Before I get any further into my suggestion, for the sake of formality the canonical form of URLs for Spotify releases is:

https://open.spotify.com/album/\{id}

...and, as such, they are whitelisted for linking onto MB releases.

Out of the major digital music vendors, Spotify is unique for only soft-deleting pages of withdrawn releases. They get hidden from searches, tracks are supplanted by recordings from other releases if a fitting replacement is found, otherwise they get grayed out.
Nevertheless, they never get properly killed-off. Anyone with the link can still access these pages, they will neither be given a "not found" message nor redirected elsewhere. Furthermore, users or tools with access to the Spotify API can still extract metadata for the releases in question.

Unfortunately MB currently lacks a functionality to make such distinctions. If you check the 'ended' box for the Spotify album URL, it will be grayed-out. Clicking on it within MB pages will be disabled, even if there's definitely no harm in visiting those links, on the contrary they can still serve for extracting or confirming info (dates, copyrights, artwork, etc.).
On the other hand, you can keep those links active, except (even if you set the status of the MB release to 'Withdrawn') they will be accompanied with captions like "Stream at Spotify" (under "External links" on the right-hand panel) or "stream for free" (when listed among other relationships bellow the tracklist) which might be misleading. Afterall, "stream for free" is the only type for URL-release relationships that gets automatically set upon adding those links to releases here on MB.

For now I'm thinking of the following two options that might help in tackling this issue:

1. Spotify album links to stay clickable even after they get checkmarked as ended (I might have missed something but as I've already said above, I'm yet to see at least one case where a Spotify release gets a proper hard-delete).
2. Add another relationship type for Spotify releases alongside the already existing "stream for free" (the name for such relationships should be up for debate, "archived on" might be suitable IMO)