Harbinger is the only post-beyond-light exotic mission not available to play, and has been unavailable for 4+ years now (removed in Witch Queen, released in Beyond Light). Despite Bungie promising to steadily revamp and re-release all exotic missions (with craftable exotics), and even teasing at the inclusion of older and non-exotic missions like Other Side and Last Rites, most of them are still missing and a Harbinger revamp (released immediately before Presage, mind you) has not happened. After Beyond Light, exotic missions have not really been "sunset" or "dcv'd", they've just been placed in the Exotic Mission Rotator and all have craftable exotics already, so we haven't had to worry about when they'll return.