How do I add a before-after slider to Shopify?

Add the OneType Transforms script tag in theme.liquid before the closing head tag. Then use a Custom Liquid section in the theme editor with two images inside a div with the ot="before-after" attribute. Free to use, no app needed.

Which Shopify plan do I need for a before-after slider?

Any plan with an online store - Basic ($39/mo) or higher. The Starter plan ($5/mo) only supports product links and does not have a full online store or theme editor.

Will the before-after slider survive Shopify theme updates?

The Custom Liquid section added through the theme editor survives theme updates. The script tag in theme.liquid may need to be re-added after a theme update. Always duplicate your theme before updating.

Before-After Slider for Shopify - Step by Step