Not really - it's not sufficiently fine-grained and, besides, you need it to connect to X to display anything. There were various attempts to improve this situ

Not really - it's not sufficiently fine-grained and, besides, you need it to connect to X to display anything. There were various attempts to improve this situation in the early 2010s, typically using Xnest or Xephyr in conjunction with other sandboxing techniques. I believe Qubes OS followed that approach but it was awkward, limited, had major performance problems, and yet never managed to fully prevent circumvention. The fact is that the X model was never designed with these threats in mind.