There is a ton of useful libraries written in C [1] [2], and I feel like there's a little revival in the last few years for using C to write "bedrock libraries", because such libraries are often easier to integrate into "foreign language" projects than C++ libraries. Those libraries also often end up in C++ projects because they are often easier to integrate into C++ projects than actual C++ libraries.

[1] https://github.com/floooh/bookmarks

[2] https://github.com/nothings/single_file_libs (here, C/C++ usually means "written in the common subset of C and C++")