Is there a way in Simplicity Studio 5 to downgrade the Bluetooth Gecko SDK (GSDK) version?

Yes, it is possible. You can for example downgrade in Simplicity Studio 5 from the Bluetooth Gecko SDK (GSDK) version 4.4.0 to 4.2.1 at any time. Refer to https://www.silabs.com/developers/gecko-software-development-kit for the latest release notes documents.

First, launch Simplicity Studio 5 and go to the Welcome / Launcher page if necessary. Next make sure that you are logged in with your Silicon Labs account – otherwise please apply and register under https://www.silabs.com. Now open the "Install" window which by default can be located in the upper-left corner of the submenu.

Go to "SDKs" and search for "Gecko SDK - 32-bit and Wireless MCUs". Click on the three dots (...) right next to the current installed version number shown and select "Change Version". Confirm your choice and GSDK release by pressing the "Finish" button. This process may take a few minutes to complete.