No. Chromebooks have all open source drivers, and you can compile it yourself if you like. The only closed source bits are third party code (usually due to patent reasons - eg. mp3 codecs).

If you want to run all your own stuff, you do need to have the machine in dev mode, which will warn you on every startup.

That warning screen is the most awful thing...

Not because of the warning, but because if you press Spacebar on that warning screen (which is the only key that the screen suggests you might want to press), it re-enables OS Verification, removing your ability to boot anything but ChromeOs until you run some more terminal commands do set it all up again..

If at any point a child, or pretty much anyone else besides you turns on your laptop, this is guaranteed to happen.

I hope they had good reason for making this design decision because for me it was one of the most frustrating aspects of trying boot outside of ChromeOS

It's to ensure that you, the user, know when the boot process has changed substantially, and that you have a simple way to get back into familiar territory.

If you want to get rid of Chrome OS and all its user protection measures entirely, that's possible with official and relatively standardized means. The open source firmware community provides documentation and tested firmware images for that (of course: no warranty), most prominently https://mrchromebox.tech/

But I don't want to get rid of it.

ChromeOS tends to provide the best experience for anything that ChromeOS is able to do.

I want to be able to choose at bootup, and not have that configuration be "easily obliterated" as someone else described. The warning is fine. The way back is a bit too simple...

I went to a lot of trouble to set up my kid's Chromebook in unlocked mode with Linux+Steam installed. It all got obliterated by one naive keypress after it ran out of battery and rebooted. That self-destruct button is ridiculously easy to push.

Yes, I had nearly the same exact experience.

Do OEMs still release modified versions of their Chromebooks with a standard BIOS?

I used to have an x86 Acer "CloudBook" that shipped with Windows and a nice EFI bios. It boots Linux well, and even has an option to disable the windows trusted boot keys, and to use user installed (i.e., grub) keys instead. The hardware seems to be designed to run chrome os, except that it has a standard keyboard.

My only complaint is that I'd like a higher-end version of it.