There was once a sport.
A common one.
Playing it invariably hurt you,

So, some clever-clogs
decided to introduce
health-promoting equipment.

The 'safety' equipment actually caused more harm and even death!!!!!!

What is the sport?

P.S no need to worry - this equipment is still keeping us 'safe' till this day!

• Is this trivia? – kaine Jul 11 '16 at 19:21
• Why the weird linebreaks - is this supposed to rhyme, or something? – immibis Jul 12 '16 at 12:40

Boxing

Reasoning:

Very common sport,has been around for a while, and usually matches were until a knockout.

Health promoting equipment:

Gloves and helmet hid injuries, so people would end up fighting for longer, and caused more permanent and long term injury

• This is my answer too, but I'd add something to your explanation. Gloves were introduced to reduce injuries, but since people could now hit harder without fear of injuring their hands, concussions and head injuries increased significantly. I believe there's data to back this up. – Duncan Jul 11 '16 at 23:00
• Gloves don't just cause boxers to fight longer and punch harder, but also encourage boxers to aim for the head. Punching someone in the skull without gloves can hurt your hand quite a lot, so boxers without gloves would tend to aim for softer areas like the chest. – BenM Jul 12 '16 at 3:27

American Football?

Reasoning:

Obviously a very physical sport, which hurts a lot. You wear helmets and padding, and its been on the news lately, and in the film 'concussion' that the helmets actually cause brain damgage

• I would also add that with greater pads, technique in tackling has disappeared, so now players tackle through violent collisions rather than technique like they were forced to earlier on in the sport – qwertylpc Jul 11 '16 at 22:17
• Good answer, and surprising similarities to the accepted answer too. – AndyT Jul 12 '16 at 13:27

“... The ‘safety’ equipment actually caused more harm and even death!!!!!!”

$\color{black}{\small\textsf{What is the sport?}} \quad$ Bicycling

This puzzle well describes bicycling in the 1800s, dangerously popular as a sport and responsible for the development of air-filled tires and modern paved roads.

(wikimedia.org)

The “safety” bicycle is basically what we are familiar with today and was introduced to allow a rider’s feet to reach the ground without dismounting, as compared to the earlier “ordinary” bicycle.

(wikimedia.org)

The incidence of injuries kept increasing anyway as ridership increased and required less training.

Some recent statistics attest to how dangerous this “sport” continues to be.