Is it still possible to install/sideload iOS apps into M1 Mac? The answer is yes - it is possible to even on the newest version of MacOS (Monterey 12.0.1).Nevertheless you have to know that not all applications are working well. For example, Instagram is crashing every time at startup, and you have to clear ~/Library/Containers/Instagram every time before launch.
How to install the iOS app?
Download and install sideloadly from https://sideloadly.io
Download application that you want to install. It can be done i.e. by iMazing or from appdb
Choose your app, enter apple ID account details (it is a good idea to use a fake one) and click on start.