In order to assist plugin devs in their testing, we have beta Qt6 versions of Sigil (for Windows and macOS) that can be installed at the same time as the regular release (Qt5) version of Sigil.
We've gone to great lengths to make sure these betas are safe, but please back up your Sigil preferences/INIs just to be safe.
Sigil-2.0.0-Windows-Qt6-Beta-x64-Setup.exe
A pre-release Qt6 Beta version of Sigil-2.0.0 for Mac (x86_64)
These betas may change from time to time.