This is a continuation of this question. It may be necessary reading to understand the context.
I'm confused by this topic too, however I think the error about "boot_partition" may have gone away? In Nov 2022, shortly before the OP, there were a couple of changes addressing Partition Manager support:
https://github.com/nrfconnect/sdk-mcuboot/commit/f7de93fe96daf9776a7331d2157d33f54e9f526f
https://github.com/nrfconnect/sdk-mcuboot/commit/82ee5d0bb9ac68c14033b102705904ee9991ee3d
As far as I can tell now (at least since ncs v2.5.2), if MCUBoot is enabled with CONFIG_BOOTLOADER_MCUBOOT, the build becomes a "Multi-image build". That enables Partition Manager, which makes fixed-partitions in the DTS irrelevant.
At least when I try it, removing all trace of fixed-partitions results in the same output, and no mention of boot_partition.
But then that makes comments like this one:
https://github.com/nrfconnect/sdk-nrf/blob/5c862041c865a7e8c6046b98608078cac852f0d0/boards/nordic/nrf7002dk/nrf5340_cpuapp_partition_conf.dts#L14
incorrect. So which is it? Does MCUBoot make fixed-partitions necessary, or unnecessary? Perhaps the answer is here, which seems to suggest fixed-partitions do nothing in ncs, but are still expected in Zephyr, so things like "slot0" are faked internally for Zephyr's benefit?