Folded header fields are deprecated as of RFC 7230; see <https://tools.ietf.org/html/rfc7230#section-3.2.4>. The only reasons to fold them I can think of is for readability (debugging), when generating a messsage/http MIME message (which I don’t think the Python library supports), or maybe when dealing with a strange server limitation. Normally there is not meant to be a limit for lines in the HTTP header, although it is common to limit the total unfolded header field value.

If we go ahead and drop folding support, perhaps we should deprecate the putheader() multi-argument mode, rather than just document the arguments are now joined by spaces. It seems a pointless API now with this change.