I want to set up a product metafield with optional multiple selection - for example, if a product has “dairy” and “sweetener”, then the “Ingredient” metafield for the product would allow me to select both “dairy” and “sweetener”, and the user filter would show this product when users select “dairy” or “sweetener”. I tried both
I can see that this used to be available in late 2022/early 2023 through the list option, like described here, but this exact setup is no longer available. How can I replicate the result? I played around with both “single line text” and “choice list (single line text)”, with no luck. I would prefer to not use plug-ins if possible.
Am I missing something? I don’t have a “List of single line text” option at all (I have Dawn 15.3, Shop 2.0). The closest is “choice list (single line text)”, which I’ve mentioned I’ve tried before. Here’s another gif showing that I don’t get to select multiple options under that content type.

If, by chance you’ve selected one of the Shopify standard metafield definitions (have namespace.key like shopify.ingredients) then you can’t modify them.
Need to create your own custom definition.
This looks exactly like what I want, but my shop doesn’t provide the min/max options for a choice list (single line text)? Here’s a screenshot of what that looks like for me. It’s identical, except for the min/max not being present, which seems to be what’s controlling the ability to select multiples. Any suggestions?
Hold on. Looks like you missed one thing.
Sorry. I should have made that clearer earlier.
Oh, I’m seeing now that your screenshot has “Type” “List”, whereas mine has “Type” “One”. So I need to change the type somehow. It doesn’t seem to be a simple click option. I’d love some pointers.
Nevermind, just found it. Huge thanks to @Maximus3 for the screenshots and help! Apparently the type (list/one) needs to be selected FIRST, followed by the selection of line / choice list.
Also as @tim_tairli said, remember that if Shopify populates suggested names, don’t click on them and don’t hit enter, otherwise you won’t be able to edit anything, unless you want that.