At a small agency I used to work at, we included a footer link for any projects we completed for a service trade or for non-profits as a way to drive leads.

I noticed in Zoe Rooney's contract sample, she has a clause for credits where removal of the footer credit is an additional cost:

We may, at our discretion, include a small footer credit on the sites that we code containing a link to our site… You may not remove that credit line without written permission from us, as long as you are using our code on your site. If you would prefer not to include that credit, please notify us in writing (email is fine) so that we can discuss the cost for removal.