What is the checkout.liquid deprecation and do I need to migrate?

Yes, you need to migrate. Shopify is retiring the old checkout.liquid file and the Additional scripts box in favour of Checkout Extensibility, checkout UI extensions, the branding API, Shopify Functions and web pixels. After the deadline, customisations living in checkout.liquid stop working. We audit what your old checkout did (custom fields, scripts, upsells, tracking) and rebuild each piece the Extensibility way before the cut-off. See our Shopify theme & Liquid bug fixing page for related Liquid work.