The unknown type is a Luau builtin type; however, it is not recognised by Roblox LSP. Using this type in any script yields the following warning:

Undefined type `unknown`. Roblox LSP Diagnostics.(undefined-type)

While ideally there should be support for the behaviour that unknown aims to implement (it cannot be used as another type, forcing type refinement), the important change is to first disable this erroneous warning.