What is the purpose of your client to inform the user that he is ADA compliant?
Is it because a compliance requirement means that they have to say that they are? If this is the case, check the requirements for ADA to display it.
Is it because they want to have it there just because they are compliant? If this is the case, the footer is suitable for this type of thing (a site similar to the website is copyright information – things that need to be said, but which are not not primary).
Is it because they want to let the world know that they comply with the ADA? (brag, brownie points, no matter how you want to call it). If this is the case, you have several options.
Make it really stand out (something that is "different" from most websites and attracts attention):
- A corner tag at the top left or at the top right
- Side label
- A sepearte top bar that can be closed with an "X" (ie like those who say "this site stores cookies")
- A pop-up
Make it more subtle (but keep it in a normally watched place):
- Under the logo of the company
- In / below the navigation bar / menu