It doesn't have to be a return value though. You could pass pointers to it as parameters.
To answer the question though, a number of people do define structures containing a buffer and a length (and potentially capacity), there just isn't such a structure standardized so everybody who wants to do this has to bring their own.
Some examples from Unix: iovec, sendmsg/recvmsg. Surely there are others I'm just not thinking of right now.
In the Windows world you have UNICODE_STRING and similar structures. SChannel has "PSecBufferDesc". Again, surely there are others.
And prominent libraries might also have their own.
Passing struct arguments by value is also complicated in some ABIs. The most reliable way is to pass a pointer to struct, but with slices that means double indirection.
I don't think ABI is a concern these days, though, even if things were different 30 years ago. The specs that we have today do cover how to pass and return structs in the standard manner.
To answer the question though, a number of people do define structures containing a buffer and a length (and potentially capacity), there just isn't such a structure standardized so everybody who wants to do this has to bring their own.
Some examples from Unix: iovec, sendmsg/recvmsg. Surely there are others I'm just not thinking of right now.
In the Windows world you have UNICODE_STRING and similar structures. SChannel has "PSecBufferDesc". Again, surely there are others.
And prominent libraries might also have their own.