There's still no plan to add sqlite support in Flarum core.
While it's technically possible to swap mysql with sqlite driver, it introduces a lot of complexities in maintaining and testing the software. We already have regular issues between MySQL and MariaDB, and those are supposed to be interchangeable 😅
If I remember correctly from reading up on sqlite support in Laravel a few years ago, it's supported by Laravel, but basically requires to not use any mysql-specific feature in database migrations and queries. I'm not sure how limited this is. Things have probably improved in recent years and I'm sure things like auto-increment, foreign keys and update constraints have some level of support/equivalent in sqlite?
Ultimately if it's too restricted, supporting it would pose additional concerns to extensions. Extensions would need to state whether they work on sqlite or not, and might create a problematic split between compatible and incompatible extensions and complicate the support work for us.
Right now we already have a hard time keeping up with the fragmented compatibility between betas, so there's no chance sqlite would be introduced anytime soon in core. And we have a lot of features planned for after stable already and I personally don't think this is useful enough to get priority.
The only situation where I see this being added to core more quickly is if it helps with our unit/integration tests. If the feature set of sqlite is sufficient to test all mysql-based core Flarum features, then this could make sense to be implemented, and therefore would also make sense to be available for end users and not just ourselves.
That being said, I totally see a possibility where a developer forks Flarum or creates an extension that enables sqlite compatibility for a small subset of features.