As on macOS (with SIP, etc.) it'll be the opposite: Users will need to opt-in to this new less-secure mode, as they do the ultra-secure Lockdown Mode.
I'm on the latest version of MacOS and I routinely install software outside of the App Store (in fact, nearly 100% of the software I install is outside the app store).
https://support.apple.com/en-us/HT202491
As on macOS (with SIP, etc.) it'll be the opposite: Users will need to opt-in to this new less-secure mode, as they do the ultra-secure Lockdown Mode.