Since when is the Mac App Store a necessity? It's still possible to download DMGs straight from the internet and install the .app by dragging it to /Applications

The only restriction to 3rd party apps are unsigned apps. Very rare these days, mainly small hobby projects. You can still activate them through the System Settings.

Unsigned apps are a pain, but you can have your app signed without being in the macOS App Store. Nearly all my apps are signed and non-App Store. e.g. Homebrew requires Casks be signed, so anything you can install via Homebrew is a single line to install without additional restrictions.