Blocking access to the “FlightSimulator.exe” file is one of the properties of the “UWP” platform. Most MS Store games are now released on the platform. If I understand correctly, the MSFS2020 is one of them. However, there is a program called “ReshadeForUWP”. Its author is “MilkyDeveloper”. It is specially designed to solve the problem. The program placed on GitHub. Unfortunately, it may not work for all users. However, there are chances of a successful installation ReShade this way.
I have devoted a separate topic to the discussion of the “ReshadeForUWP” program in the section on third-party developers. In the topic, I talk in detail about the program and my experience of using it. The manual that I mentioned above is published in this topic.
Here is a link to the topic.