It like looks several CouchDB features are deprecated in 3.0 and will be removed in 4.0. Is this functionality gone or just moved somewhere else?
Removed functionality (per: https://docs.couchdb.org/en/latest/whatsnew/3.0.html#deprecated-feature-warnings)
Show functions (/{db}/{ddoc}/_show)
List functions (/{db}/{ddoc}/_list)
Update functions (/{db}/{ddoc}/_update)
Virtual hosts and ini-file rewrites
Rewrite functions (/{db}/{ddoc}/_rewrite)
How are rewrites handled in 4.0?