ESC is the industry name each auto maker had their own name for their version of what we call ESC today.
Subarus first ESC cars were offered with what Subaru called VDC.
GM called theirs StabiliTrac.
You see where this is going right? Subaru was putting their stability control system in their Top models starting in 2001. Like all tech it has it's own evolution over time. VDC was sold as a feature in higher end models till 2004. By 2005 all Subaru models had drive by wire throttles called ETC. ETC paired with ABS enabled Subaru have what you know as ESC. Each auto maker chose how active or intrusive to make their ESC. Toyota got its hand slapped not long ago for not making their ESC active enough in their high end SUVs. Subaru cars by the nature of their design do not need or require highly active / intrusive ESC to keep their cars right side up.