In 1.21.8 or below, supported_formats could be either a number, list, or object. Starting from 25w31a, this field can seemingly only be a list. I believe this change was unintentional as there was no mention of it in the changelog
I have attached two packs which both load correctly in 1.21.8, however in 25w31a the pack using the object format (longform_formats) appears as “broken or invalid” despite having the correct format versions specified. I’ve also attached both packs below alongside screenshots
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As I mentioned, there was nothing in the changelog to state that the old format has changed, and if you want to make a multiversion pack you need to use this old format anyway. A breaking change like this being present without any mention in the changelog is unusual, which is why I don’t think it was intentional
Mojang has deleted “supported_formats” and community developers need to use “min_format” and “max_format” instead.