The (old-fashioned Central European) pressure cookers I've worked with have outlet valves which in pressure cooking position let gas out (at defined pressure), but not [easily] in. These cookers do have an underpressure after cooling which you manually relieve before opening (it's plain impossible to move the lid unless you do that). Either with a lever pushing the rubber sealing or by twisting/screwing the valve into an "all directions open" position. I don't know whether/how long they keep the full bar of underpressure, but after some days you still get the "ffflump" sound when air goes in and the rubber detaches from pot wall and lid.

I'd certainly not expect this to last years like with a jar, but I do use it for several days or maybe a week. Whether you do, is up to your own judgment.

There's also the question of what is in there: keeping fruit from molding until I have time to finish and jar the jelly a week later => fine. Personally, I also don't have a problem with, say, a goulash kept that way since (and iff) it's properly heated again before eating (the 2nd heat treatment will destroy any botulinum toxin that may have formed - such a twice-cooked-scheme is btw an officially recommended option over here). For everything else, I think the probability of anything I cannot detect by sight and smell getting in without any detectable microbial contamination is negligible. Note that this is the same heuristic as for canning: underpressure OK and no mold, no smell, no bad taste => everything as it should be.

I wouldn't keep fish that way - but then I don't pressure cook fish anyways.

Note also that

• the official food safety recommendations for private homes over here contain more "know which food is prone to have undetectable problems" and more "trust your cerebellum" for the rest.
• The local climate would typically allow me to have a closed pot with cooked quinceys stand in a place of, say, 10 - 15°C. Which is not as cool as the +8°C of the fridge, but also quite different from the "never below +30°C" I had in summer in my kitchen when I was living in Italy.

The (old-fashioned Central European) pressure cookers I've worked with have outlet valves which in pressure cooking position let gas out (at defined pressure), but not in. These cookers do have an underpressure after cooling which you manually relieve before opening (it's plain impossible to move the lid unless you do that). Either with a lever pushing the rubber sealing or by twisting/screwing the valve into an "all directions open" position. I don't know whether/how long they keep the full bar of underpressure, but after some days you still get the "ffflump" sound when air goes in and the rubber detaches from pot wall and lid.

I'd certainly not expect this to last years like with a jar, but I do use it for several days or a week. Whether you do, is up to your own judgment.

There's also the question of what is in there: keeping fruit from molding until I have time to finish and jar the jelly a week later => fine. The risk here is to have to throw away the food and work, but it's not a risk of not being able to detect if that stuff went bad.
Personally, I also don't have a problem with, say, a goulash kept that way since (and iff) it's properly heated again before eating (the 2nd heat treatment will destroy any botulinum toxin that may have formed - such a twice-cooked-scheme is btw an officially recommended option over here).

For everything else in terms of microbiological contamination, I think the probability of anything I cannot detect by sight and smell getting in without any detectable microbial contamination is negligible. Note that this is the same heuristic as for canning: underpressure OK and no mold, no smell, no bad taste => everything as it should be.

I wouldn't keep fish that way - but then I don't pressure cook fish anyways.

Note also that

• the official food safety recommendations for private homes over here contain more "know which food is prone to have undetectable problems" and more "trust your cerebellum" for the rest than what I know about North American recommendataions.
• The local climate would typically allow me to have a closed pot with cooked quinceys stand in a place of, say, 10 - 15°C. Which is not as cool as the +8°C of the fridge, but also quite different from the "never much below +30°C for weeks" in summer when I was living at the Mediterranean Sea.
  (My latitude in North America would translate to the north end of Newfoundland or Vancouver Island, or Regina)