The function fills in the struct. Maybe it then sends the struct via a packet-oriented protocol, perhaps over a datagram socket, so the length is transmitted implicitly in the packet size.
That’s different from a VLA, which doesn’t use dependent typing or any reliance on users keeping variables unmodified to guarantee they clean up the stack properly.
There is no problem cleaning the stack because the compiler has numerous ways to do that. One way is to keep a copy of the original n, as passed to the function. Another way is to save the original stack pointer.
Making this work is not much harder than supporting the alloca function.
Then you have to read part of the file, read some size fields, make a struct on the stack (for an image file??), copy the stuff you've read in already into that struct, and keep reading. And you now need to define how to pass a pointer of that struct type to helper functions for operating on a PNG.
For example, a parameter n is passed to a function, and then a struct is defined based on n.
The function fills in the struct. Maybe it then sends the struct via a packet-oriented protocol, perhaps over a datagram socket, so the length is transmitted implicitly in the packet size.