DearIMGUI has both an unofficial C wrapper and a new official C binding generator. I have personally used the CIMGUI wrapper with C99 apps using the Direct3D11 backend and it works great. The new API is started by the DearIMGUI creator but is still adding features that are available in CIMGUI.
Unofficial C API - https://github.com/cimgui/cimgui Official binding gen - https://github.com/dearimgui/dear_bindings