If I recall (need to Google it but am out and about at the moment), Safari won’t evict if you’ve installed it as their version of a progressive web app (share button -> “add to Home Screen”).
(I say “their version” because they still lack a number of things most PWAs use, like more robust service workers for background operations, notification support, etc)
A few months ago, I decided to play around with IndexedDb (for the first time) and PWAs and created a Stable Diffusion web interface that stores data in your browser. [1]
I have thousand of images stored as base64 strings in Safari on iOS since the beginning of October and it’s all still there!
Granted… this is a web app installed as an “app” on my iPhone’s Home Screen. So, IndexedDb inside Safari itself may still behave as you say.
It was recently written up in PC World [2] as part of an article about a distributed cluster of Stable Diffusion instances [3].
If so, Indexeddb should only be used to store “unsaved” files for safari.
Chrome and Firefox support https://caniuse.com/mdn-api_permissions_persistent-storage_p...
But that implies if the user denies persistent storage, then indexedb should be again only used to store “unsaved” files