This is extremely common in the embedded and systems programming space, which Rust is otherwise attractive to use in.
For example, a very popular library in the systems/embedded space, cjson, works with C89. FreeRTOS works with C99. This is a very common pattern in major libraries. It is very rare that taking a dependency could force you to update your toolchain in the embedded space.
Toolchain updates invalidate your entire testing history and make you revalidate everything, which is often a giant PITA when your testing involves physical things (ex you may have to plug and unplug things, put the system in chambers that replicate certain environmental conditions, etc).
