I only recently became aware of Landlock. It does not appear to have seen wide traction, although it remains an example of an LSM coming closer to a real capabilities model.
Google very quietly dropped support for their port of Capsicum to Linux around 2020.
I only caught wind of Sandstorm.io's seccomp-bpf filters late last night when diving on the code. As ever, it's just a syscall ACL mechanism, though hats off to Kenton for reminding us all that Linux "capabilities" are not capabilities.