Summary
There is a function in the legacy catalog experience in which you can add multiple values to a field by using a line break (Example URL path: https://mystore.vtexcommercestable.com.br/admin/site/CampoValorForm.aspx?idCategoria=123&idCampo=456&flagSku=0#).
The UI says you can add multiple values by using a line break, such as: "Value A Value B Values C"
And this would add three distinct values. However, this usability is not currently functional. The UI is refreshed upon attempting to do so, and a "Try this Tip!" message is shown instead (which gives no useful information).
The data itself is not saved.
This may not affect every store.
Simulation
1 - Access the field value UI for the legacy catalog (Categories --> Fields (either product or SKU) --> Values --> Add values)
2 - Try adding multiple values via a linebreak, such as the example below:

3 - You will not be able to, and a "Try this tip" message will be shown.
Workaround
There are multiple workarounds:
1 - Add these fields one by one using the same UI 2 - Add them via API 3 - Add them via sheet import