If we want to gradually migrate our clients from v1 to v2, shouldn't we have a pref on the client instead, and do there what you do here in the devtools:

const apiVersion = Services.prefs.getCharPref("api_version", "/v1");
const serverUrl = new URL(SERVER_URL, apiVersion);

We haven't really found a design that would allow us to gradually roll out clients to v2

The changes of this PR work to try things out on nightly or when running the client with MOZ_REMOTE_SETTINGS_DEVTOOLS=1. But won't be able to also support our api version migration later

A potential idea would be to use a separate preference on the client:

Services.prefs.setIntPref("services.settings.api_version", 2);

And on the client have something like:

let apiVersion = Services.prefs.getIntPref("services.settings.api_version");
apiVersion = [1, 2].contains(apiVersion) ? apiVersion : 1;

and do the concatenation on the client code