Strictly speaking, the first third-party OS for Clickpad calculators, DummyOS, was made in 2011, as a companion project for OSLauncher

But a port of Linux + userspace, assuming we reach that state, would be the first
usable third-party OS for Nspire calculators. DummyOS is just a PoC, but it's completely useless in practice

Being the author of both OSLauncher and DummyOS, I get to criticize them - especially DummyOS - as much as I want ^^