Tl,dr: use an if-statement in Express middleware to redirect based on user feature flags.

Yawn and flagged.

EDIT:

Also, it looks like there may be a syntax error in their handler. Comments are mine, added:

/* pretty sure that you need an = sign on your lambda arrow, folks */
const featureFlaggedRouting = (req, res, next) > {
  /* what, I guess Node has an `If` library now? */
  If (req.user.featureFlagExists) {
    /* whoa tiger...Express middleware functions shouldn't be returning anything, least of all more middleware! */
    return newRoutingLogic(req, res, next)
  }
  /* are we meaning to early-out above, or is this a bug? */
  next()
}

Here, maybe you all will find the docs useful. :|

Maybe try:

const featureFlaggedRouting = (req, res, next) => {
  req.user.featureFlagExists? newRoutingLogic(req, res, next) : next() ;
}

Note that, without an idea of what newRoutingLogic should be, my refactor could be incorrect.