Glancing at Wikipedia, all hydraulic shock absorbers seem to use pneumatic compression. The oil is mostly a mechanical linkage, lubricant, and heat sink. I expect a liquid-only design could work, in a coilover monotube, but the spring would be taking all the compression, while the loose piston moving through oil simply resists change and smooths out the motion. There’s just not much reason to avoid adding a floating piston and some gas at the bottom of that. Underwater applications, maybe.