I’m curious what you mean by this — why does coding for length prevent streaming? The receiving end can certainly treat the text as a stream still. Do you mean that the sender cannot “stream” if they are generating the contents on the fly? That seems trivial to solve - just break it up into chunks, assuming a little bit of buffering is OK.
Commenter above, correct me if I’m misinterpreting, but — I imagine they meant for when programs talk to other programs. Regexes and string literals are written by humans; it would be easy for the compiler/program to calculate the length for you.