It is technically possible, yes. You can turn Gatekeeper off via the command line in various ways, or even via an obscure deliberately non-discoverable set of GUI tricks.
But it isn't reasonable to expect any normal person to do that. So, in practice, any app that isn't some open source widget targeting developers does register them with Apple. In this sense it also isn't possible.
This isn't specific to ARM. It's also been true on Intel Macs for a long time too. The only thing that changed on ARM is some minor detail - the kernel now requires a "signature" for all binaries, but a "signature" is also allowed to be a hash match against a local machine-specific whitelist, so this doesn't make much difference in practice to anyone except toolchain developers. It seems to have mostly been about reducing tech debt in the security stack.
The registration process is however very lightweight. There are no app policies involved beyond "don't distribute malware" and "verify your ID so we can do something about it if you do". It's not like the app store where there are lots of very subjective criteria. To get an identity is nearly automatic, you can do it as an individual with a credit card and approval is automated. Ditto for applications: it's automatic and driven by a simple (albeit undocumented) REST API. You upload a zip containing your signed app to S3, it's processed automatically, the app now works. The notarization API is extremely open - you need an API key, but otherwise anyone can notarize anything, including apps written by other people. So in the early years of this system when lack of notarization just triggered a security warning, lots of people notarized any app they found that was missing it. This made a nice smooth backwards compatible path to transition the ecosystem. Nowadays, there is no bypassable security warning, an unnotarized app is just described as corrupted and won't open without tricks.
So - does macOS "support" sideloading or not? It's very ambiguous. You can argue both yes and no.