Why do I get a juttering, vibrating, or shaking at low speeds when I release the clutch? If I give it an excessive amount of throttle/gas while I release the clutch it does not do this.
Maybe your clutch is going bad. Since you didn’t tell us anything about your Subie, this is just a guess. More details will get a better guess. Post back with how many miles on this thing, has the clutch been replaced before, any check engine lights, any leaks - oil-water-brake fluid-clutch fluid ect. and we’ll try and help a bit more.
We are also assuming you are in first gear. That car has relatively poor torque at low engine speeds, so if you are rolling slowly and try to use 2nd that may happen. I suspect that is not the case and Mustangman has answered.
Glazed clutch disc, glazed flywheel, faulty pilot bearing, clutch throw-out bearing snagging on a worn transmission mainshaft snout are some possibilities. Any of those means a clutch job.
Way too little information.
OP comment “excessive amount of throttle”, could this be an engine rather than clutch issue.
All we know is it is a 21 year old vehicle. Might be long overdue for service. Ignition, plugs, wires, etc and or fuel delivery, to low compression.