My checkout customizations stopped working, is this the checkout.liquid deprecation?

Almost certainly. Shopify has retired checkout.liquid, so legacy checkout edits, additional scripts and order.status customizations stop running. The fix is to migrate to Checkout Extensibility, UI extensions, Shopify Functions, web pixels and the new branding API. We audit what you relied on, rebuild it the supported way, and verify the checkout end-to-end. For a deep dive see our Shopify checkout bug fixing page.