Can I alter your last but one sentence to add one word??
The vast majority of the kernel is INITIALLY written in a high-level language. And this is the modern-day role of assembly language programming in most use-cases, but, not all.

Also most drivers will be written in Assembler.
Though C or oone of it's derivatives is used a lot now.

Assembly language is still used. In addition to the links already provided, a new compiler (and porting a compiler to a new environment) often starts with assembly of a core compiler that is then used to compile a more complete compiler, etc. to bootstrap the new system development environment. Obviously not everyone is doing work that requires an assembler, but just as obviously you can do things using machine language and assembly that are difficult to do any other way.

Moreover, many higher level languages have features designed to call an assembler against provided code to enable functions, interfaces, or controls that are not supported by the base language definition.

For embedded systems in particular, assembler is very handy. In systems running FORTH it is common to assemble the core about 2K of code) which includes primitive compiler and assembler hooks that allow you to then extend FORTH to any extent required. Even today some embedded systems operate on a very restrictive memory footprint, and well crafted assembly results in machine codes that is very small and efficient.

as a matter of fact yes, if you go to www.ticalc.org or www.cemetech.net, they have a repository of assembly language programs for various z80 and mc68000 based TI graphing calculators, still being developed.

please keep in mind it is just a fork of MenuetOS.

Great OS, seeing your link.

Thought I mentioned that.