Yes, the phoenix server was shut down a few months ago but unfortunately, we haven't gotten around to fixing the guides yet to remove or update that SchemaSpy URL. We plan to do this as part of this issue:
I should note that
https://guides.dataverse.org/en/5.4/schemaspy/ was a one off manual build of SchemaSpy for Dataverse 5.4. So it represents the database for that version of Dataverse. I look at that 5.4 SchemaSpy output all the time but I'm conscious of the fact that it doesn't reflect the latest database schema, since it's always changing in small ways.
I don't know if this is in scope for the issue above but ideally we'd have automatic builds of SchemaSpy for each future release as well as a build for what's in the "develop" branch (the latest, unreleased code).
Hope this helps,
Phil