IndexedDB is a web storage API. It provides a persistent database that can be used to store large amounts of data on the client-side, accessible even when the client is offline.
When Lockdown Mode is enabled on iOS, the IndexedDB API is disabled. Apps that rely on IndexedDB would need to switch to a different storage mechanism to regain functionality delivered by that API.
IndexedDB is otherwise supported on all major browsers, including Chrome, Firefox, Safari, and Edge. It is fully supported on iOS through Safari, starting with iOS 10.