According to both Google and Google, if I enable Linux on Chrome OS, then in Settings' "Linux (Beta)" section, I should see a "Develop Android apps" option.
On an HP Chromebox G2 (Kench), I see this option and eventually got it activated.
On a Samsung Chromebook Plus (Kevin), I do not see this option, even with Chrome OS 88.0.4324.186.
On an ASUS Chromebook Tablet CT100 (scarlet), I see this option. This particular device is short on ports, though, so I'm going to be struggling to use it for what I need.
On an Acer Chromebook R11 (cyan), I do not see this option, even with Chrome OS 88.0.4324.186.
So... how do I enable ADB debugging on the Samsung or the Acer?
Or, another way to look at it: how do I know whether a given Chrome OS device model will or will not support ADB debugging, besides actually trying to set it up? It is difficult to plan on getting something else for testing if I do not know whether it will support ADB before buying it.
It's a known issue since September 20.
There is no official response from google about this issue yet.
According to Google's product platinum expert DennyL:
"I believe it has to do with the architecture and/or the kernel version used on the device" ......... "So far, there have been no authoritative responses but we hope to see something soon."
Source: https://support.google.com/chromebook/thread/71852070?hl=en