There is no jailbreaking. You are given root access out of box[2]. They don't discourage external developers, they even provide a toolchain[1] to build for the device upon request. Hell, their CTO at the time (not sure if they still are) wrote some non-official software that people can install[4], and helped some of the initial members of the community get their stuff working. They just don't provide official support for external software on the device.
As for bricking your device: It will not hard brick, it may only soft-brick[0], as in, the display will not update on the rM2, because the software driving the screen needs to be updated to support the new OS. You can still SSH in and revert the changes and then downgrade. If you were silly enough not to write down your SSH password, or set up an SSH key and can't get into SSH, you can still recover your device[3] on your own.
As for bricking your device: It will not hard brick, it may only soft-brick[0], as in, the display will not update on the rM2, because the software driving the screen needs to be updated to support the new OS. You can still SSH in and revert the changes and then downgrade. If you were silly enough not to write down your SSH password, or set up an SSH key and can't get into SSH, you can still recover your device[3] on your own.
0. https://remarkable.guide/faqs.html#can-i-install-toltec-befo... 1. https://remarkable.guide/devel/toolchains.html 2. https://remarkable.guide/guide/access/ssh.html 3. https://remarkable.guide/tech/recovery.html 4. https://github.com/sandsmark/recrossable