Since the N900 was based on Linux it is pretty easy for the community to work with what we have. The downside is we haven never had open source hardware driver support from Nokia or the upstream manufacturers, this is why we only can do, and have done, so much in the kernel to backport new security and tech into said old kernel.
Nokia has long abandoned this 'burning platform' and actually sold it's rights to build phones to MSFT for something like 5 years, though they are now making android phones. Everything here is all community now.
If you want a new phone where the community is building the OS, including a project building a new version of the Maemo OS found on the N900(some of the best software for mobile but it can't make phone calls yet) there is the Pinephone(most available OS options are still super alpha as in dev-only grade for OS & software) they just made a keyboard clamshell cover available last week. There ARE FOSS drivers for all Pinephone hardware so it will have a vastly longer life as a frontline device, though I have been using a N900 as my primary for about 10 years but am now testing Pinephone and am a dev for add-on hardware.