You're right, the update seems to have changed the Shift key behavior for locking aspect ratios. A possible workaround is to check if any new settings were introduced that might restore this function. Otherwise, using the Properties panel to lock the ratio manually is the best option for now.

There doesn’t seem to be a new setting, so hopefully it’s just a consequence of the recent update and they can fix it soon.

Hope so, though, I have read about somewhere else as well but only support can help us in this process.
Regards Mate!

​@EclipseStar the same thing happened to our team. Our entire icon library had the “Constrain Proportions” turned off when the Figma team changed to “Aspect Ratio Lock”. 

ALSO, the Aspect Ratio Lock does not automatically lock when hold SHIFT to draw a proportionate square, circle or any other object. 

Using the SHIFT key still does not lock aspect ratio. 
ALSO, when Figma updated from the “Constrain Proportions” to “Aspect Ratio Lock” every object that previously had “Constrain Proportions” set/locked was cleared and not replaced with the “Aspect Ratio Lock” enabled. Soooooooooo DUMB!!!!!

Yes, it's quite disturbing.
Additionally, after enabling aspect ratio scaling, it will not be possible to set horizontal and vertical constraints for scale at the same time. 

Hi everyone - just wanted to chime in and say that we did bring this up with the teams that work on aspect ratio, and they are continuing to collect feedback.

While I myself don’t have any kind of date or estimated timeline of potential work, we’ll try to get updates to you as they come.