Percentage of earnings is just equity. They're different, but not ethically. Slavery would be forcibly taking 100% of an individual's equity, but given that ISAs are both optional and a minor percentage (Lambda's was 18% when I went thru), the comparison is unreasonable.

If the percentage of earnings is capped (as it was in most agreements), it's far better than a loan-- worst/best case you pay the capped amount (like the loan amount); if you do worse than that best case, you pay proportionally less.

But trying to claw earnings from jobs that didn't relate to the school violates the letter and spirit of the agreement and shows the disproportionate power of the parties.

Okay, so what's an income based repayment plan for a loan?

I believe they're capped to a maximum time and fixed amount, so it's like a loan where the payments depend on your revenue no? And if you don't reach the max amount during the max time you end up paying less.