Coming back to this as Projects functionality is deprecated, but this remains unresolved.
Seems the idea for referencing linkable objects is now to use / as done in Slack, etc. See Ability to use # character to help reference any linkable object on the Nextcloud instance · Issue #31667 · nextcloud/server · GitHub
A functionality such as projects, is still needed to at least be considered in some manner. It is a worthwhile conversation to continue fwiw.