Until there's a Linux OS that's "blessed" by Apple there's a choice you will have to make. 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. The problem is that with the T2 chip enabled only signed operating systems will boot. The problem with the T2 chip and running Linux has little to do with rEFInd. This ability to run multiple operating systems also comes with the speed, low cost, and durability of modern 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. I now routinely run one or two virtual machines on my newer computers. There's also been considerable advancements in virtual machine software. 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. 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. 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.
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. There are other scenarios for booting Ubuntu.
A driver is installed by rEFInd that allows the firmware (and thus rEFInd) to read linux formatted partitions.There is also option with rEFInd to do a Grubless boot of Ubuntu. Ubuntu is also made the default operating system to boot. The shim boot loader boots Grub from the EFI partition.
The shim boot loader is booted from the EFI partition by the firmware.The user chooses Ubuntu from the Startup Manager.The user holds down the option key at startup until the firmware boots to the Mac Startup Manager.Grub boots Ubuntu from a linux formatted partition.The firmware boots GRUB from the EFI partition.rEFInd instructs the firmware to boot Grub.The user chooses Ubuntu from the rEFInd menu.Installing Ubuntu to the internal drive of the newest Macs may not yet be possible. If you lower the security, then you can install and boot with or without using rEFInd. So with secure boot enabled, you can not install Ubuntu. Also, you can not boot the Ubuntu Live USB flash drive to install Ubuntu. If you employ all the security offered by the T2 chip, then you can not boot Ubuntu.