Short answer: yes. Here's the fine print, what you actually get, and how to know whether your flight has it.
Yes — United Starlink WiFi is free for MileagePlus members, and MileagePlus itself is free to join. No purchase, no elite status, no data caps.
On board, connect to the WiFi network and sign in with your MileagePlus number (or join free on the spot). It works gate to gate — no waiting for 10,000 feet.
Free Starlink is only on Starlink-equipped aircraft — 563 of 1,658 United Airlines aircraft today, with more added weekly. Aircraft still awaiting installation mostly carry an older system — Viasat, Panasonic or Thales — which is slower and usually paid, and some carry no WiFi at all. The fleet page shows which is which, tail by tail. Whether your flight has Starlink depends on the aircraft assigned, so check your flight number and date — the answer is verified near departure and updates continuously.
Curious how fast the gap is closing? See the rollout timeline.