"I found out that Amazon gated these ASINs without any prior notice and doesn't accept applications to sell."

Happens all the time on here. As @Seller_OvL8C4BJWiuS9notes, do you have an LOA from the brand that says you are permitted to sell on Amazon?

"I always source these products directly from the manufacturer."

Do you mean that you source from their wholesale side, OR are you buying from a public facing website run by the brand?

"Why did it happen? "

Most likely due to this policy change --

DIETARY SUPPLEMENTS

https://sellercentral.amazon.com/seller-forums/discussions/t/08220707-e6c4-41d6-b135-0bb521f5bc7a?postId=08220707-e6c4-41d6-b135-0bb521f5bc7a

From @manny_amazon on 4/4/2025 --

As part of our ongoing efforts to provide the best possible customer experience, Amazon has implemented additional restrictions for Ingestible Brands, a.k.a dietary supplements. Unfortunately, we are not accepting applications for ingestible brands at this time.

If you feel that the categorization is wrong, please reach out to Seller Support so they can investigate this for you. If you are able to get this re-categorized, please provide me with the new case ID so I can look into this further for you.

If you are a brand owner, you can also reach out to Brand Registry Support and ask them to approve you within the Health & Personal Care category, which will allow you to list those products.

Regards,

- Manny

"Could you please allow me to sell out these items, and I will not replenish them anymore?"

NOT likely unless you have/get an LOA from the brand and then you should be OK. Are you selling FBA? If so, do a product removal so you get your items back so you can sell elsewhere. If you are doing FBM sell someplace else. You should have been selling on multiple channels anyway for protection so this just forces the issue for you.