It has always been supported and this is the syntax. It doesn’t support mixing types though (llvm obviously support that just fine but we just don’t have a syntax for it…) I believe this is documented last time I check…
Note that on x86 (64bit only iirc) vararg floating point is one of the few cases this is needed. The calling convention requires the number of fp vararg to be passed in rax so without the ... the result may be wrong.