Reflexion will be End-of-life on March 31,2023. See Sophos Reflexion EoL FAQs to learn more.
When my license of Intercept X Advanced expires what happens?Will I keep receiving updates?Am I still protected?
So after the grace period, the protection part will automatically deactivate and uninstall itself and and Microsoft Defender (Windows environment, obviously) will kick in?
I believe this to be correct, although I'm not entirely certain how Windows Defender will react. Some information on defender can be found in the following article. - learn.microsoft.com/.../microsoft-defender-antivirus-compatibility
I'd suggest testing this first. You can do so by using the "Manage Endpoint Software" button from Sophos Central. Removing all protections from one test device will give you the same results as though a licence expiry were to occur.