I have recently upgraded to Catalina on my old 2015 Macbook Air. I needed to upgrade as a couple of pieces of software as they would no longer allow upgrades and they would not sync across the iOS devices.
After the upgrade was completed Text expander would no longer work, this had quite an impact to some of the workflows I have. However this is a simple fix.
I could list all the steps, but it is easier to follow the simple instructions from the people that make the app.
https://smilesoftware.com/blog/entry/textexpander-macos-10-14-mojave-and-accessibility/