That's cool, but then I'd assume you have to make an index for each function. In general if a person is asking basic questions about sargability, I will steer them away from functions on a predicate (it's usually my last choice.)

Often you can just think a bit harder about your set based logic and make an index that's reusable by other queries.

In other SQL's you can also make a materialized view, which may be able to persist some of these things, its just like keeping n+1 copies of your data to satisfy a different set of relations/projections.

SQLite has it too (caveats, like always, apply): https://www.sqlite.org/expridx.html.