The full story is much more complicated, but briefly, the switch to 64 bits offered a chance to revisit the kernel implementation,

but the version that was closest to plan 9 was incomplete on amd64, and never released except for powerpc where most of the work was done,

and the other forks that were released for amd64 were more experimental (eg, Nix). A year or two back a further attempt was made

to retrace and set out again, because so much old crud had got added back in,

but that stalled when several of us ran out of time or got too busy. At the time, none of us was interested in adding yet more unfinished

or unpolished software to the big cloudy bitbuckets.

I'm sure at least one of those will be back soon.

There was, however, a parallel effort to allow a more conventional licence, which did finally prosper.