Any kind of library that does a lot of low-level stuff (kernel syscalls, custom binary reprs, ffi) will have to use unsafe. But most applications built using these libraries rarely need to use unsafe at all, because the libraries act as safe wrappers to make sure the app developer isn't accidentally violating invariants allowed by the "unsafe" keyword.
Yeah. For example Lemmy backend is written purely in Rust and doesn’t use unsafe anywhere, but some lower-level library it uses probably does use unsafe for IO code.