Finally, a GNU/Hurd phone! /s

Any “modern” wifi will require a firmware blob. But if you are ok with older wifi chips like ath9k, then you can avoid the blob.

Do you think ChromeOS could fit that role? At least it shows that an alternative to Windows exists.

It is not open source, because it does not meet the definition of open source.

I’ve noticed that some “mobility” startups are using Linux. E.g. companies working on electric or automated vehicles.

Perhaps disconnecting the RF antenna, and replacing it with an appropriate termination?

Yes, but does that still count as “modern”?