For historical reasons, references on artifacts (art and artifact keywords) was checking this existence of the targeted artifact to decide to render a reference as a link or not. This behavior was specific to tracker v3 and was ported to tracker v5 but never applied to other references (doc, git, etc).

In the context of Tracker v3 removal, it no longer make sense to keep the behavior for several reasons:

• it's inconsistent. Tuleap should behaves the same, if writting doc #1231231241412314 transforms to a link even if the doc 1231231241412314 doesn't exist, then writting art #1231231241412314 should do the same.
• one could arg that we should align on resource verifications (ie do not convert link on non existant doc references). That's not really possible because:
  • some references are very resource intensive to verify (like git references)
  • external references cannot be verified (like your_own_tool #123)
  • resource verification breaks the permission model. A resource that is not accessible to someone should not be different from a resource that doesn't exist