wpler

<a href="https://www.wpler.com/linkmanager-fur-multisites/" rel="nofollow ugc">Deutsche Beschreibung des Plugins</a>
You run a Multisite Blogsystem with open registration for users to add own blogs and their can using a theme by 
different installed themes, but you want to show a link into the footers of the themes but only once time and for 
every blog another one?
With this plugin you can manage your outgoing links into the themes by a shortcode/function for every blog. You can 
set for every link the target, relation (Attribute rel), linktext and -title and the target url. Also you can set more 
than one link for a blog.
For output you can use a shortcode or a function . This function get the links 
for the current blog-id and printed this, if links are available.
All options/links can set by network admin only. The outputs needs no capability.

WPMU LinkManager

Manage your Themes footer links for every blog into a multisite includes with conditional tags.

upwerd

Remove or replace the footer credit of any WordPress theme with the click of a mouse.

Visual Footer Credit Remover

Remove or replace footer credit of any WordPress theme with the click of a mouse.

WPMU LinkManager vs Visual Footer Credit Remover