I saw some on my last visit to the arrivals lounge a few months ago. I've never seen anybody using it (but then I usually just shower and head into the city).

Lately, I've been taking the morning flight to LHR, which arrives long after the arrivals lounge is closed, so I can't tell you if it's been removed as a cost cutting measure

If you want a workout, go through FastTrack Immigration, go to the Arrivals lounge, workout, shower, go downstairs to Heathrow Express and on to Terminal 4. If you have 4 hours to kill, you'll get to the BA lounge in plenty of time.