Are there any FOSS OS with macOS-level security?
macOS have superior security compared to standard Linux/BSD distributions even at the software level, separate from their hardware lockdown.
Are there any FOSS desktop OS with equivalent security, including proper verified boot and exploit mitigations? If not, why?
Apple's approach to verified boot is pretty intense: https://support.apple.com/guide/security/boot-process-secac7...
Recently I learned about an interesting approach to this through a presentation at CCC about a small project known as sixos. Go to the part at 18m27s where ownerboot is presented: https://media.ccc.de/v/38c3-sixos-a-nix-os-without-systemd
Repository for ownerboot: https://codeberg.org/amjoseph/ownerboot
Yes, Fedora Silverblue would be my recommendation for something balancing security but being mainstream enough.
If you want more mainstream you could go with vanilla Fedora or some spins if you are opinionated about your desktop environment.
If you want more security you can look at things like QubesOS.
> macOS have superior security compared to standard Linux/BSD distributions even at the software level
What evidence do you have to state that claim?
> including proper verified boot and exploit mitigations? If not, why?
Any operating system with TPM and Secure Boot support has this, Windows, Linux, and FreeBSD is in the planning stages for Secure Boot.
It sounds like you're unaware of the available security features that have been available for a significant amount of time on x86.
There is a huge gap between what's available for Linux (ie the Android and ChromeOS boot systems), and what the mainstream Linux distributions actually implement.
On the whole, the distros do a terrible job. If you can modify the initrd (which you can on all of them by default), it's game over.
And that's just secure boot. Linux has no real equivalent to TCC, for example.
Qubes OS, security-oriented operating system, has a much higher security than MacOS: https://qubes-os.org.
Some elevator pitches: https://forum.qubes-os.org/t/how-to-pitch-qubes-os/4499/15