Actually, there is but not In the way you expect. When the M1 Mac’s we’re released, you could sideline iOS and iPadOS apps but that was blocked by Apple in a later update. On the surface this is a means of clamping down on app piracy and unauthorized use of an app but also a a blockage all the same.
Being able to run arbitrary code doesn't really mean you get to run other people's code without their permission, which is essentially what running those applications on a mac was. You can argue the merits of DRM and copyright of course, but ultimately that's not the same thing as "running arbitrary code." You can run the code and that code includes a check to make sure the platform is one that the developer approved of. If you find a way to remove that check it probably still runs.
Not adopting technologies at the rate of the market leader who is pushing for more and more control over the web might be "stalling web innovation" but the alternative universe is one where Chrome has no meaningful check against its hegemony of the web and can freely create new technology in support of Google's mission to harvest private information and serve advertisements to its users.
> Sideloading in this case is eliminating choice
There is nothing on MacOS that is preventing installing and running arbitrary code on your machine. Elevation of privileges is required for some of that code, but it is there for those that need or want it and a sane default exists for everyone else. This is the case with every modern operating system, including free ones.