If you want to find devices that still need hardware support under Linux, I highly recommend trying to get a mobile Linux distribution to work on an old smartphone or tablet.
postmarketOS in particular has a really good devices page [1] showing missing feature support at a glance, as well as guides for porting to new devices [2] and porting features from an outdated vendor-provided Linux fork to the upstream kernel [3].
postmarketOS in particular has a really good devices page [1] showing missing feature support at a glance, as well as guides for porting to new devices [2] and porting features from an outdated vendor-provided Linux fork to the upstream kernel [3].
[1] https://wiki.postmarketos.org/wiki/Devices [2] https://wiki.postmarketos.org/wiki/Porting_to_a_new_device [3] https://wiki.postmarketos.org/wiki/Mainlining