When a product's remaining stock gets down to 0, the item will appear crossed out on your form with the message "SOLD OUT." You can change this message by creating a custom Translation.
Open the "Theme" menu by clicking the droplet icon in the top-right corner of the form editor.
Select the "Translations" tab, then click "Add Translation +."
Enter a Translation Title and select a Translation Language.
Even if you're simply changing the wording in English, you must click and select "English" from the dropdown menu, or you will not be able to save the translation.
Scroll down to "SOLD OUT" and enter the text that you'd like to replace it with.
Scroll all the way down to the bottom of the list of UI Elements, and click the "Create Translation" button.
Find your new Translation in the list and click the "Use" button to activate it for this form.
If you'd like to apply this change to all forms on your account, you can head to your Account Translations menu, find your new Translation, and select "Make Default."
Gotchas
If you see a button labeled "Please fill out all required questions" where "Create Translation" should be, this means that either the Translation Title or Translation Language is missing. The grey text "English" that appears in the Translation Language dropdown by default is a placeholder, not a pre-selected answer. You must click this, and manually select "English" (or your language) to proceed.
You may already be using a custom Translation (e.g. if your form is not in English, or if you've changed the wording of other UI elements). In this case, edit the existing Translation rather than creating a new one.