P: Doesn't constrain proportions when resizing from side handle while holding Shift key

Of course not, because screwing long time pro users is the Adobe Way.

Is there a way to turn this feature off?

@Hannah Nicollet i'd like to know Adobe's reasons for reversing shortcut keys and command behaviors overnight. Who thought this was a good idea? We now need to coach all of our users on the new behaviors, and build custom preference files for others. On each machine we use. Unnecessary and obstructive. 

I am so, so annoyed by this update. I spent 5 minutes trying to understand if I had pressed some sort of button that was canceling out Shift or if my Mac was being glitchy. Then, I remembered I updated Photoshop, so I came here to find out this is a thing that the developers did on purpose. What kind of monsters remove the Shift function without sending out a mass warning to let users know they are fundamentally changing how one of the most used scale methods work? 

This is the stupidest update ever. Don't hold shift anymore, Adobe are you serious? I also teach this - don't hold shift in all programs but Photoshop. Lets all have to unlearn something we don't even think about doing since the creation of Photoshop. Just dumb.

I wonder will they take us seriously and made such option in preferences with next update or leave with way of .txt file to do the same. I only hope they levae us choise which method we want to use.

PLEASE make a revert to legacy in preferences on this topic. You have inconsistencies amongst your products now. Illustrator and AE (and others) are using the old method and Photoshop is now using this new method. 20+ year photoshop user here... I can adapt but it's super nice to have the option to revert to my old ways if possible. Plus, I teach high schoolers the Adobe package and consistency is key. Thank you. 

The most ridiculous compatibility they did I still can't understand that has to do with mentioned other Adobe applications is that they did in CC 2018 and kept in CC 2019 that made me to stay still in CS6 Extended. I have no idea how that what's said in first reply to following topic is so important taking into consideration that you work in Photoshop with many documents while in Illustrator or InDesign rather with one at once. They forced us to some nonsense behaviour in Ps only that was (not explained what exactly!) the same with other applications for some reason, but did anyone thought how much that change affected work in Photoshop? JEA, you was shocked like me:

https://feedback.photoshop.com/photoshop_family/topics/photoshop-cc-2018-ctrl-tab-not-bringing-up-mi...

(...)Alt + Shift to scale from center has also been lost because of it.

Excelent point!

AMAZING! Thank you for the help 🙂
This update makes sense.