Change:
size of the quoted packet can actually be smaller, or the information unintelligible, as described in Section 1.1 of DPLPMTUD. <del> QUIC endpoints SHOULD validate ICMP messages to protect from off-path injection as specified in RFC8201 and Section 5.2 of RFC8085.  This validation SHOULD use the quoted packet supplied in the payload of an ICMP message to associate the message with a corresponding transport connection DPLPMTUD. </del> <ins> QUIC endpoints using PMTUD SHOULD validate ICMP messages to protect from off-path injection as specified in RFC8201 and Section 5.2 of RFC8085.  This validation SHOULD use the quoted packet supplied in the payload of an ICMP message to associate the message with a corresponding transport connection (see Section 4.6.1 of DPLPMTUD). </ins> ICMP message validation MUST include matching IP addresses and UDP ports RFC8085 and, when possible, connection IDs to an active QUIC <del> session. The endpoint SHOULD ignore all ICMP messages that fail validation. </del> <ins> session.  The endpoint SHOULD ignore all ICMP messages that fail validation. </ins> An endpoint MUST NOT increase PMTU based on ICMP messages; see Section 3, clause 6 of DPLPMTUD.  Any reduction in the QUIC maximum

Discussion:
This is with some minor additional changes: Reflow. Move the odd text about using Handshake back where it belongs. Make the language about how to construct a probe less prescriptive. Mention 1200 when referring to the "smallest allowed maximum packet size" so it is clear what that means.
NAME I went with "PMTU probe packet", which is an IP packet. Let me know if that makes sense.
OK - There are already "IP packets" and "QUIC packets" - so some other term works fro me.
A top-level comment on the term "probing": In Section: Probing a New Path: A packet containing only probing frames is a "probing packet", and a packet containing any other frame is a "non-probing packet". Please be sure that "probe" can not be confused with "probing" in PLPMTUD.
In the subsection on “Packet Size {#packet-size}”, I think the blocks of text describe two things. There is some general guidance on size, and there are specific instructions for the size of the initial packet. I’d suggest we do try to separate these two, it would make it much easier for someone to understand the size constraints for forming the initial packet.
Regarding the sizing of Initial packets, see . I checked the text for probing and we have "PMTUD probe" in one case and "probing packet" elsewhere. It's not ideal, but I didn't see any cases that were clearly busted. I don't have a good idea for alternative labels, but would welcome suggestions.
It turns out that coordinating the publication of all the documents is a real challenge. Right now, DPLPMTUD depends on QUIC, which is fine, but it turns out that this is a problem for other things that depend on DPLPMTUD who now transitively depend on QUIC. QUIC could adopt the DPLPMTUD text that is specific to QUIC and loosen this dependency chain. The text changes are relatively modest, see . While this could be editorial, I think that this will require a consensus call. The nature of the changes is such that it should be treated as design. Also, I think that there are substantive changes required.

So from an AD perspective, draft-ietf-tsvwg-datagram-plpmtud is currently blocking another document (URL) from being published that has a missref on it. Except for this question of QUIC I am ready to approve draft-ietf-tsvwg-datagram-plpmtud. To me it is also clear that integration of DPLPMTUD done in the QUIC WG will result in less editorial clarity issues that potentially could make an future implementer uncertain. As can already been seen in the PR there a few question that potentially can improve the operation and not unnecessary restrict the operation of DPLPMTUD. From my perspective it looks like the core is a set of minor editorial issues to resolve that will make the DPLPMTUD be clearer and resolve the missref issue and function as well as keeping it in the TSVWG document would. If a little bit more effort is invested after the first step then there is potential for additional improvements.