Code change:
integer value (represented as a variable-length integer) that represents the maximum size of a DATAGRAM frame (including the frame type, length, and payload) the endpoint is willing to receive, in <del> bytes.  An endpoint that includes this parameter supports the DATAGRAM frame types and is willing to receive such frames on this connection.  Endpoints MUST NOT send DATAGRAM frames until they have received the max_datagram_frame_size transport parameter.  Endpoints MUST NOT send DATAGRAM frames of size strictly larger than the value of max_datagram_frame_size the endpoint has received from its peer. An endpoint that receives a DATAGRAM frame when it has not sent the max_datagram_frame_size transport parameter MUST terminate the connection with error PROTOCOL_VIOLATION.  An endpoint that receives a DATAGRAM frame that is strictly larger than the value it sent in its max_datagram_frame_size transport parameter MUST terminate the connection with error PROTOCOL_VIOLATION.  Endpoints that wish to use DATAGRAM frames need to ensure they send a max_datagram_frame_size value sufficient to allow their peer to use them.  It is RECOMMENDED to send the value 65535 in the max_datagram_frame_size transport parameter as that indicates to the peer that this endpoint will accept any DATAGRAM frame that fits inside a QUIC packet. </del> <ins> bytes. The default for this parameter is 0, which indicates that the endpoint does not support DATAGRAM frames.  A value greater than 0 indicates that the endpoint supports the DATAGRAM frame types and is willing to receive such frames on this connection. An endpoint MUST NOT send DATAGRAM frames until it has received the max_datagram_frame_size transport parameter with a non-zero value. An endpoint MUST NOT send DATAGRAM frames that are larger than the max_datagram_frame_size value it has received from its peer.  An endpoint that receives a DATAGRAM frame when it has not indicated support via the transport parameter MUST terminate the connection with an error of type PROTOCOL_VIOLATION.  Similarly, an endpoint that receives a DATAGRAM frame that is larger than the value it sent in its max_datagram_frame_size transport parameter MUST terminate the connection with an error of type PROTOCOL_VIOLATION. For most uses of DATAGRAM frames, it is RECOMMENDED to send a value of 65535 in the max_datagram_frame_size transport parameter to indicate that this endpoint will accept any DATAGRAM frame that fits inside a QUIC packet. </ins> The max_datagram_frame_size transport parameter is a unidirectional limit and indication of support of DATAGRAM frames.  Application

Related discussion:
text: For streams you have an entirely different mechanism, and other frames are generally not large.
NAME that's an interesting thought - is this something you're planning on implementing and have a use for, or is it more of a thought experiment?
NAME Unfortunately I've had to push back on my QUIC dev, but I did work with an emulation of netmap, on top of select / poll, that uses slots to receive packets from the OS without context switching. Sometimes a packet needs to span multiple slots, but often it just drops into a 2K slot or so. I could imagine many use cases where datagrams are redistributed to other threads or processes in a similar manner without having an explicit use case. I believe there are now several kernel interfaces that uses a similar approach.
As far as I know, those kernel interfaces all use slots that are larger than the interface MTU, so I don't think they'd require reducing the payload size. Am I missing something?
Well, I can only speak for netmap, but it works by having the user specify one large memory block divided into a number of equal sized slots of a user specified size. Each slot has a control record with some flags. A record can indicate that the slot is partial and reference the next slot in the packet. So you can can choose a slot of 64K and waste a lot of memory, or choose a slot of 512 bytes and frequently have to deal with fragmenting. Or chose 2K for a use case where you know that you will not have larger payload and can afford the space overhead. EDIT: to clarify, it is the network driver that chooses to link data and update the control record when receiving, and the user process when writing. I'm not sure exactly how the other kernel interfaces behave but from memory it is rather much the same. For QUIC datagrams it is a bit different in that you likely want to avoid fragmentation at all cost due to the added complexity and the non-stream nature of the data. If you could afford fragmentation it would make less sense to have a limit on the size.
I support switching to TP specifying max payload size.
IIRC, the original reason this limit was introduced is to allow QUIC proxies to communicate the max datagram size it gets from the MTU between itself and the backend.