That is clearly not true. People entretain making changes to the protocol all the time. Bitcoin is far from perfect and not improving it would be stupid in my opinion.
Some improvements require changes to the consensus rules.
Recent changes include relative lock time verify or segwit. These are important changes that made things like lightning much easier and efficient than they could possibly be without them.
Taproot, which is a recent proposal, could help simplify the lightning protocol even further, and make it more efficient and its usage more private. And there are more use cases.
There have been consensus rule changes since bitcoin started, and with good reason. As a user, you can always oppose new changes. And if enough users agree with you, you will be able to maintain your own chain with the old rules. At the same time, there's nothing you can do to stop other users who want those changes from coordinating with each other to adopt them.
Perhaps you're interested in bip99, which discusses consensus rule changes in more detail.