https://webaudio.github.io/web-audio-api/#dom-audionode-channelcountmode says:

ScriptProcessorNode
The channel count mode cannot be changed from "explicit" and an InvalidStateError exception MUST be thrown for any attempt to change the value.

Testcase:

var ctx = new AudioContext
var n = ctx.createScriptProcessor(2048, 3)
try { n.channelCountMode = "max"  } catch (e) { console.log(e.name) }

logs "NotSupportedError" in both Chrome and Firefox.