Which Edition of ECMA-262 Does Google Apps Script Support?

The documentation says that the old runtime is based on Mozilla's Rhino JavaScript interpreter which provides a subset of ECMAScript 5 and is based on 1.6, with a smattering of 1.7 and 1.8.

The new Apps Script runtime is supported by the V8 runtime with few exceptions like E6 Modules.

From Built-in Google Services

Apps Script supports two JavaScript runtimes: the modern V8 runtime and an older one powered by Mozilla's Rhino JavaScript interpreter.

The V8 runtime supports modern ECMAScript syntax and features. The Rhino runtime is based on the older JavaScript 1.6 standard, plus a few features from 1.7 and 1.8. You can freely choose which runtime to use with your script, but the V8 runtime is strongly recommended.