It happens basically any time I search with at least three tags/filters. Someone else mentioned it here: https://itch.io/t/4848013/free-in-browser-idle-429-error?before=4#post-12799330
It started roughly a week ago (give or take), and since then search with 3+ tags/filters has basically never worked, even if it's the first time I'm hitting the site that day. I don't doubt it's a DDoS protection measure, but there seems to have been a configuration change about a week ago that set the threshold to trip even on a fairly standard search.