If you want to install applications over UART, you'll need some kind of system for receiving those bytes and writing them to memory. That's usually the tock-bootloader. Rarely, some systems have a hardware bootloader system capable of that. The tock-bootloader repo is not obsolete. It's just been quite stable for a while now, so it hasn't needed updates.
Alternatively, many platforms Tock supports have USB->JTAG support. This doesn't require any software like a bootloader, but it does require hardware to support it, usually in the form of an additional chip on a board. That separate chip can be an entirely separate Microcontroller running software like DAPLink, which is how the Microbit v2 works, or some other hardware JTAG converter.
JTAG support is preferred over bootloaders when available, since it doesn't require any additional software running on the actual microcontroller.
Thanks,
Branden Ghena
Northwestern University