How to dual boot Ubuntu and macOS without rEFInd?

I recall being able to install Linux with Boot Camp but that was some time ago, my recollection may be incorrect and Boot Camp support from Apple has been fading in recent years. Support for Boot Camp will end with the end of Intel based Macs, meaning not much effort has been put into keeping it updated, and there’s nothing I’ve seen to suggest an equivalent technology for future ARM based systems.

Is there a reason to dual boot as opposed to using a virtual machine? I gave up on dual booting a very long time ago. In that time computers got cheap enough, and reliable enough, that I can afford keeping an old computer (or seven) around for running whatever OS I want and avoid the need to reboot. There’s also been considerable advancements in virtual machine software. I now routinely run one or two virtual machines on my newer computers. This let’s me flip between Linux, macOS, and Windows with a key macro instead of having to reboot or go to another computer. This ability to run multiple operating systems also comes with the speed, low cost, and durability of modern computers.

The problem with the T2 chip and running Linux has little to do with rEFInd. The problem is that with the T2 chip enabled only signed operating systems will boot. I don’t know if Apple bars Linux from being signed, nobody bothered to go through the process of signing the OS/kernel/whatever, or why the two don’t meet.

Until there’s a Linux OS that’s “blessed” by Apple there’s a choice you will have to make. You can boot Linux with the T2 chip at lowered security settings. You can run Linux in a virtual machine. You can not run Linux on this computer. If there’s another choice then I’m not aware of it.