For systems programming, C89 is definitely a "sweet spot". It's relatively easy to write a compiler for and was dominant when system variety was at its highest, so it is best supported across the widest variety of platforms. Later C standards are harder to write compilers for and have questionable features like VLAs and more complicated macro syntax. C++ is a hideous mess. Rust is promising, and would probably be my choice personally, but it's also still fairly new and will limit your deployment platform options.
C89 is still a reasonable choice today. I don't think it's depressing. It's a good language, and hitting the sweet spot of both language design and implementation is really hard, so you'd expect to have very few options.
C89 is still a reasonable choice today. I don't think it's depressing. It's a good language, and hitting the sweet spot of both language design and implementation is really hard, so you'd expect to have very few options.