Hey Erik,
Welcome to Apple Support Communities. It sounds like you may have had a shortcut set up for quickly creating a new sticky note in Stickies that no longer works after upgrading to macOS Catalina. I have a desktop dedicated to dozens of stickies that help keep me organized. I can relate to wanting to ensure that it works correctly!
I wasn’t familiar with the shortcut, so I cross-referenced the shortcut you were using with the keyboard shortcuts outlined here:
Mac keyboard shortcuts
I’m not seeing the Shift-Command-Y option here, so is it possible that you created your own shortcut? This article outlines that process:
Create keyboard shortcuts for apps on Mac
When you follow the guidance here, do you see the shortcut in System Preferences > Keyboard > Shortcuts? If not, what happens if you try to add it?
If you see it there, it’s possible that it may be conflicting with another shortcut:
If a keyboard shortcut doesn’t work on Mac
Keep in mind that Apple lists their shortcuts as “Shift-Command-Y” instead of “Command+Shift+Y”. This may impact your search.
Hopefully these articles help you resolve the shortcut issue. Being able to quickly open a sticky note with a keyboard shortcut seems like it may be incredibly convenient.
Take Care!