Major versions that change basically everything should be considered different projects and require a re-review. I remember I did this early with react-router, and I think when they changed the public API for the second time, we forked it and left it as-is, because they made their goals apparent.

If Svelte version 4 and 5 are very different, then they're "SvelteV4" and "SvelteV5", both on version 1.0.0, in my mind :)