The Shopify Plus requirement is the one hard platform boundary in this product, and it's worth separating from everything else that looks like a limit but is actually just a plan tier. Checkout offers and checkout customizations require Plus because Shopify itself restricts checkout UI extensions to that plan — no UpsellPlus tier change gets around this. Every other surface — product page, storefront pages, cart in all its variants, post-purchase, thank-you page, customer accounts, and POS — works on any Shopify plan, including the free Basic tier.
Plan tiers beyond that are gated by monthly upsell revenue generated (net of discounts, same definition used in analytics), not by which features are turned on — the free tier covers up to $500/month, and each paid tier raises that ceiling along with adding capabilities like checkout access (Plus-gated separately), multivariate testing, and managed service. Crossing a revenue ceiling triggers an automatic upgrade to the next tier rather than disabling anything that was already running, so an offer that's performing well won't suddenly stop working because it crossed a threshold.
A headless storefront is the other situation worth flagging explicitly: checkout-extension-based placements are unaffected by headless setups since they run inside Shopify's own checkout regardless of how the storefront is built, but storefront-rendered contexts (product page, cart, cart drawer) depend on the theme app block being present, which a fully custom headless frontend may not include without additional integration work.