Last Updated: December 9th 2020
Many people use the Tumblr infinite scrolling script by Cody Sherman, but his page is dead and the script itself has also stopped working with a Tumblr update. I made a very small tweak to it and now it appears to work just fine.
If you are adding infinite scrolling for the first time, just add the script tag below to the bottom of your theme body. Or swap out the old script tag for this one if you already had it in your theme.
<script src="https://matthewrayfield.github.io/tumblr-infinite-scrolling/infinitescrolling.js"></script>
If it stops working please let me know.