How do you know when something is too loud without measuring it? If your answer is "ringing ears", you've already incurred at least some hearing damage.

Do you have the tools to measure how loud your headphones are? I don't. And so I keep them as low as I can.

Is there a limiter in amplifying headphones to keep them from going above a certain threshold (like 80Db)?

Why would you be trying to find the point at which it is too loud? The goal is to amplify things to the point that you can hear them clearly, not to go beyond that.

Also hearing aids have manual volume control which a licensed professional has no influence over, so that doesn't really make sense as a justification for limiting availability.

It's certainly possible to apply volume limiting to headphones, though again the people using amplifying headphones typically are dealing with the opposite problem.

When you have hearing loss you also have decreased loudness threshold levels, which will be freq specific. It is important that you do not exceed these levels, both for comfort and for speech understanding.

hearing loss is far more than just "make it louder".