CI on legacy systems?

I am currently investigating methods to make my code compatible on as many devices as possible for sake of code quality.

Is it possible to implement DroneCI to build and run runtime tests of my software on legacy systems alike VAX, minix, lispmachine, etc… ?

Drone is written in Go, and Go supports the following OS targets:

aix android darwin dragonfly freebsd hurd illumos ios js linux nacl netbsd openbsd plan9 solaris windows zos [1]

The operating systems you listed in your post are not supported by Go and therefore would not be natively supported by Drone. The only option I can think of is to run qemu inside a Docker container to emulate your target operating systems, however, this is not something I have any experience with.