Free Transform and Shift No Longer Constrain Proportions in Photoshop

You might not be using the most recent version of Photoshop. When they first changed this behavior, there was no option for going back in the newest version, you're able to revert this to the Legacy behavior.

Can you tell us the version # of the Ps you're using?

Hi. If you don't have the "Use Legacy Free Transform" option, maybe you don't have the most recent version. You can check in the Creative Cloud app if there is an available update. At this time, the latest Ps version is 20.0.5. 

I was told to go check a box that says "Use Legacy Free Transform" under settings > general, but I don't have that option

You have to be running version 20.0.5 released in June 2019 to see that option. Earlier versions (20.0.0 to 20.0.4) don't have it.

To check the version, click Help > System Info in Photoshop and read the first line in the box.

Learn how to access and read your Photoshop system info.

See New and enhanced features | Latest release of Photoshop​

(scroll to "How do I turn off the new proportional scaling by default behavior while transforming layers?")

My personal computer is the newest OS with the most recently updated Adobe CC Photoshop and it works fine.

It's either running CC 2018 or version 20.0.5 of CC 2019.

What actual version do you have? Please don't say "latest", we're looking for a number like 19.3.45. The reason "latest" is no use is that often problems are caused by updates not happening. Please let us know if you need help getting the number.

Also please tell us your actual system version. Please check it.

Constraining proportions is now set by default. You need to use SHIFT only if you do not want to constrain proportions.

To get back to the old way, open Preferences, General Settings and check this: