> Has anyone defined a strict subset of C to be used as target for compilers? Or ideally a more regular and simpler language, as writing a C compiler itself is fraught with pitfalls.

The main reason you'd target C is for portability and free compiler optimisations. If you start inventing new intermediate languages or C dialects, what's the benefit of transpiling in the first place? You might as well just write your own compiler backends and output the machine code directly, with optimisations around your own language's semantics rather than C.

Imho, C89 is the strict subset that a compiler ought to target, assuming they want C's portability and free compiler optimisations. It's well understood, not overly complex, and will compile to fast, sensible machine code on any architecture from the past half century.