FAPA.aero sent a hiring update email today (18 Oct 2022) which contained this quote. Source unknown, but sounds accurate based on replies in this thread.

"Horizon Air ratified a new pilot contact that provides numerous benefits including industry-leading pay rates for Horizon pilots throughout all steps of the seniority list. Horizon is not currently hiring direct First Officers but those interested in joining the team can look into Horizon’s Pilot Development Program and Ascend Pilot Academy. Both programs provide stipends (up to $26,000) and mentorship to flight student during their time-building journeys. In exchange for the support, members of this program agree to fly for Horizon upon completing ATP minimums. Both the Ascend Pilot Academy and the Pilot Development Program aim to break down financial barriers and empower the next generation of aviators."

I would interpret that as demand for FO's is decreasing so Horizon is only selecting FO's from their PDP or Ascend programs - and even that sounds very slow.