I was recently asked about and I-864 (Affidavit of Support), and how it could be extinguished.
Can you have a signed & witnessed indemnification that you cannot sue or require financial support from a person who legally sponsored your green card application?
No. The I-864 form is a commitment between the sponsor and the government, not the sponsor and the individual sponsored. Because there could conceivably come a time when the government compels the sponsor to pay for the person sponsored, you cannot waive that requirement (it is against public policy.)