Okay, before I go with the whole story telling, here is your answer if you reached this page searching for “how to”.
How To Enter Entering Developer Mode on ARM Chromebook
Hold down the ESC and Refresh keys and poke the Power button. To enter Dev-mode you first invoke Recovery, and at the Recovery screen press Ctrl-D (there’s no prompt – you have to know to do it). It will ask you to confirm, then reboot into dev-mode
Here is the complete info from the Chromium developer page, along with the reason why they got rid of the dev switch.
On this device, both the recovery button and the dev-switch have been virtualized. Our partners don’t really like physical switches – they cost money, take up space on the motherboard, and require holes in the case.
To invoke Recovery mode, you hold down the ESC and Refresh keys and poke the Power button.
To enter Dev-mode you first invoke Recovery, and at the Recovery screen press Ctrl-D (there’s no prompt – you have to know to do it). It will ask you to confirm, then reboot into dev-mode.
Dev-mode works the same as always: It will show the scary boot screen and you need to press Ctrl-D or wait 30 seconds to continue booting.
You’ll still have to run “
crossystem dev_boot_usb=1” and reboot once to boot from USB drives with Ctrl-U.
To leave Dev-mode and go back to normal mode, just follow the instructions at the scary boot screen. It will prompt you to confirm.
If you want to leave Dev-mode programmatically, you can run “
crossystem disable_dev_request=1; reboot” from a root shell. There’s no way to enter Dev-mode programmatically, and just seeing the Recovery screen isn’t enough - you have to use the three-finger salute which hard-resets the machine first. That’s to prevent a remote attacker from tricking your machine into dev-mode without your knowledge.
An unrelated note: Holding just Refresh and poking the Power button hard-resets the machine without entering Recovery. That’s occasionally useful, but use it with care – it doesn’t sync the disk or shut down politely, so there’s a nonzero chance of trashing the contents of your stateful partition.