What is the purpose of your client notifying the user that they're ADA compliant?
Is it because a requirement for being compliant states that they need to say that they are? If that's the case, check the ADA requirements on how to display this.
Is it because they want to have it on there just because they are compliant? If that's the case, then the footer is fine for that kind of stuff (similar to the website is copyrighted information - stuff that needs to be said, but isn't primary).
Is it because they want to make the whole world know that they are ADA compliant? (bragging, brownie points, whatever you want to call it). If that's the case, then you have several options.
Make it really stand out (something that is "different" than most websites and calls attention):
- A top-left or top-right corner tag
- A side tag
- A sepearte top bar that can be closed with an "X" (ie. like the ones that say "this site stores cookies")
- A pop-up
Make it more subtle (but keep it in a location that still normally gets looked at):
- Underneath the company logo
- In/under the navigation bar/menu