Elias here with Shopify Support. Thank you for reaching out.
I understand that your out-of-stock products are no longer appearing in the online store. Were there any changes to the theme's settings, theme's coding, or new apps installed recently?
Certain themes have an option to show or hide out-of-stock products in the Storefront editor. However, depending on the theme, this can be in various locations. You'll need to check the 'Collection pages', 'Collection lists', or 'Theme settings'.
Here's an example of what that function may look like:
With that being said, I'd be happy to take a closer look. Can you share an example/screenshot of this from your store?