This footer comes on public shares (share-links) and consists of

    the simple signup link that can be disabled with the config variable

     'simpleSignUpLink.shown' => false,

    in config.php

and

    the part that you can manipulate yourself with the theming app.

If you want to remove the footer COMPLETELY, you must manipulate the file:

lib/public/AppFramework/Http/Template/PublicTemplateResponse.php

where you have to change the line 39:

	private $footerVisible = true;

in

	private $footerVisible = false;

You can do that with this sed command, where variable NC_DIR must be assigned to the path of your installation first:

NC_DIR="/var/www/nextcloud"

Completely remove footer:

sudo sed -i 's/private \$footerVisible = true;$/private \$footerVisible = false;/1' $NC_DIR/lib/public/AppFramework/Http/Template/PublicTemplateResponse.php

Make footer visible (default):

sudo sed -i 's/private \$footerVisible = false;$/private \$footerVisible = true;/1' $NC_DIR/lib/public/AppFramework/Http/Template/PublicTemplateResponse.php

You need both, to reset it to the default, when you want to run intigrety checks. After that you can switch it back.

Hope that helps,
much luck!