At least some of them do appear to support sideloading, for example my TCL 43" from 2019. I haven't tried loading anything yet, but following the "Enable Developer Mode" process described in the Roku docs [1] does yield the option to enable the dev server, and I don't know that there's any reason to suspect IDK apps won't run if loaded. (Might need to update the OS, though; I haven't done that in a while.)
All apparently first-party by the product names, though, which mine isn't.
I'm going to try building and sideloading one of the IDK sample apps today. If it works, I'll update with details including the TV make, model, and OS version; if not, I'll...just update with that, I guess.
0 for 2. The first-party stick with OS 10.5 gives an "IDK applications are not supported on this device" dialog; the TCL TV, with OS 10.0 and no update available, just black-screens and stays that way till I hit Home.
I'm not really sure what the point is here, if I'm honest. Why even bother releasing an SDK with such limited device support, especially pitched at hobbyists who can be assumed to have (and want to support) the widest device range of any developer segment? It's not as if I'm going to buy new hardware just to look at my calendars and other day-planning info over coffee on my kitchen TV rather than my phone...
This is unfortunate. I was only interested in the IDK because roku is built in to my tv.