@OP and others, here's how to enable this flag temporarily.

In the pretty printed version of https://www.redditstatic.com/desktop2x/Reddit.de2e3f279d82ee... add a breakpoint to the first line of this part of the code.

    if (!as(e))
        return;
    const t = e.user.account ? Jt()(e.user.account.id).toString() : void 0
      , s = document.createElement("script");
    s.src = Object(es.a)("https://s.udkcrj.com/ag/386183/clear.js", {
        dt: "3861831591810830724000",
        pd: "acc",
        mo: 0,
        et: 0,
        ti: $t()(),
        ui: t,
        si: "d2x"
    }),

Then when you are paused on the breakpoint evaluate this in the console:

  e.runTimeEnvVars.staging=true