I wasn't sure were to post this, but I thought it might help some people who are using Moodle 4.0 and want to revert back to the old checkbox layout. I've managed to achieve this using CSS and I've attached my CSS code with comments explaining what each line does. I'm using the Adaptable theme, so if you use a different theme, you'll need to update the URL for the checkbox images.
These customisations work for me because I am only using 'automatic activity completion' in my courses, so I'm not sure how it would work with activities that need to be manually marked as completed by students. But perhaps others can build on this CSS code to achieve that.
I should also mention that when you have multiple criteria set for one activity (e.g. for a quiz, if you have set 'User must view the quiz' and 'User must achieve a passing grade'), you would end up with two checkboxes next to the one activity. So a workaround for this is to hide the icon for the first criterion, and only show the checkbox for the final step you want the user to complete. I was able to do this by finding the relevant activity module numbers using the Inspect tool in my browser. Let's say my quiz was module 5 in the course, you would apply the CSS:
#module-5
.badge:first-child {display: none;}
Luckily, I only had a few activities in my course, so I was able to easily hide these one by one, but I see how this might not be a practical solution if you have lots of activities with multiple criteria across many courses.