If the airplane stall behavior is no longer on par with the certification requirements then its no longer airworthy.
Exactly!
I've done hundreds of maintenance check flights, and I always stall the plane a couple of times, before signing off on it. Trainers in particular must stall correctly, or I won't sign. If you're flying a trainer which does not stall and recover as it should, that's a snag, which requires rectification.
Though tread drift, I had occasion to flight test a 3 hour since new (so brand new) bushplane. I was flying it to assess an ski installation for approval. Stalls were a part of the assessment. No matter how carefully I entered the stall, the plane spun viciously. I landed, and reported the defect, suggesting that the plane should be returned to the manufacturer, to find out why it went 'round half a turn in a stall no matter what. The owner's pilot acknowledged this advice from me. A week or so later, prior to getting the plane back for the suggested conformity inspection, the same pilot stalled and spun after takeoff, killing himself and a passenger. My report played a role in the investigation. Other's of the same model plane were checked, and indeed, I flew another a month of so later, and it was a pussycat. It took more than a year to find the defect, but it was found, and no other non compliant planes were produced. It was found that the plane I had flown had not been stall checked on the type acceptance flight test. That pilot, sadly, died in a stall spin after takeoff of a different type a few years later.
Stall characteristics must be checked post
any control/rigging maintenance!