Hi!
Like @gracewitter, this issue almost makes me crazy!
The method add_filter, did not worked for me anyaway.
Then, I found another solution, because I wanted modify, but not remove all credits.
1. Using the same philosophy of child themes, create folders:
customizr-child\templates
customizr-child\templates\parts
customizr-child\templates\parts\footer
2. Copy footer_credits.php, from parent cusomizr theme, to footer child folder. (customizr-child\templates\parts\footer)
3. Edit the child footer_credits.php as you wants.
4. Don’t forget activate your child theme.
I hope that this solution help others users.
Regards,
Alex.
