What happens when you try to assign { as a shortcut for the action via Edit > Preferences ?

AZERTY keyboard, maybe?
(a known behaviour in this keyboard configuration)

As always, sample score and steps to reproduce the problem would be useful. Also, say what you have tried changing it to - it could be indeed that you just happened to pick another combination not recognized as expected by your OS / Qt. To be sure, try more custom shortcuts, including ones you have verified do work when assigned to other commands.

Which measure of measures were you selecting when decreasing stretch? I didn't find any problem with any of the measures I tried, but I did have to decrease about 30-40 notches before I saw anything happen, because you've increased the stretch by equally crazy amounts. Go to Measure Properties to see the stretch values you have defined - the default is 1, you have it all the way up above 4 in many measures, and the step value is 0.1.

Best t simply select all then reset the stretch to start over whatever you were trying to do.

Please read my explanation above in full. You have increased stretch to such an extreme degree, you will need to reduce it by up to 30 or notches before you will see any effect, but it's working exactly as it should. It was a mistake to have increased stretch so much - once you increase it so much that only one measure fit per system, the additional increase did nothing but make it harder to reduce later. That is why I think you should start over, because the score is in a very messed up state right now that far longer than it should to correct. But if you wish to do so, start by reducing stretch 30 notches.

Does the attached file work for you?
Select measures 1-11 (all those with notes).
Reduce stretch once; measure 10 should now jump back to system 1.

Please go to Edit → Preferences… and in the shortcuts tab press the Save button.
Attach the resulting shortcuts.xml file here.

I don't have a Mac, but I know the code well enough, and can see what's going on in your score clearly enough, that I am still quite confident I am right.

So, please humor me and try the following:

1) open the score
2) reset the shortcut to the default, or set it to something you have verified works for other commands (and tell me what it is)
3) right-click measure one
4) Measure Properties
5) confirm you see the entirely inappropriate value of 4.20 for Layout stretch
6) OK
7) with the measure still selected, press your shortcut exactly 20 times
8) go back to Measure Properties
9) check the value of Layout stretch

If I'm right - and I'm quite confident I am - you will see the stretch is now 2.20. Still insanely high, which is why your reducing of stretch hasn't worked yet.

Now, reduce stretch 12 more times (there could be font differences between systems here, maybe it will take 13 or 14. You should finally have reduced stretch enough to actually cause the next measure to fit on the first system. And if you go to Measure Properties, you'll see the value of "Layout stretch" is now 1.00, which is where it would have been had you followed my advice to reset stretch in the first place :-)

All of this is exactly as it should be given the completely inappropriate amount of stretch that was already preent. It really does take 32 presses to get it back to normal.

BTW, please do not change the status of this (or any) issue from "needs info" until a developer is able to confirm and reproduce the problem.

To be clear, for others following the discussion or who might land here later: it is virtually certain it was never broken. Try the precise steps I listed above with the attached score and you will see it is exactly as I said. It is important people realize this, because it is easy to get yourself into a situation where it appears the shortcut doesn't work, but I assure you, it does. and always has. The steps I list produce exactly the same results in 3.2.3 and development builds of 3.3 as well as every previous MuseScore 3 release.

That being said, reducing stretch often seems to work much less often in 3.x than it did in 2.x (at least if you select multiple measures worth, i.e. a whole line in order to make place for just one more measure), or at least really differently…

@mirabilos the stretch algorithm is different to be sure, but probably more to to the point, so is pretty much everything else about layout, so it could be the passage in question simply takes more space than it might have in a previous release that, for instance, allowed collisions, or didn't reserve space for lyrics hyphens, etc. Feel free to start a new forum discussion with a specific score for us to look at.

i have the exact same problem

Had been fixed though, back in 3.3

I have QWERTY and can't increase the stretch with a shortcut. The symbol that is suggested in the drop down menu is not on my keyboard. That would be the curly bracket.

The curly bracket is usually the shift of the square bracket ] or [. Also, if you need to, you can redefine the shortcut in preferences.

Well, I'll be a monkey's uncle. That done it!

I have the exact same problem

Then update to 3.3.0 or (better) newer