• This was originally tracked in (Feature Request) Terminal Environment setting (like in VScode) microsoft/terminal#2785
• it was added in Support environment variables in the settings microsoft/terminal#15082
• and that first shipped in Terminal 1.18

And you can enable it like so:

The profile's environment completely replaces the one specified in defaults.

So if you've got:

"defaults": {
  "environment": {
    "Foo": "Bar"
  }
},
"list": [
  {
    "name": "PowerShell",
    "environment": {
      "Baz": "Qux"
    }
  }
]

Your PowerShell profile will get Baz=Qux and your cmd profile will get Foo=Bar.