Supporters of net neutrality warn that the end of the Obama-era rules will spell doom for internet freedom. Rather than treat all web traffic equally, telecom companies will be free to carve up the web into slow and fast “lanes,” costing consumers more time and money, they say.

Ajit Pai, chairman of the Federal Communications Commission, has played down the fears and said there are still rules in place to punish companies which harm consumers.

The...