Best regards,
to your question "is anyone actively using XML as a data transport format?" I hope the answer is negative, because it is easier for the transport of data to use JSON. ;)
Another thing is to continue using XML vocabularies, e.g. UBL from oasis-open.org, to generate electronic documents, which have dematerialized commercial documents and especially commercial documents for electronic businesses; the evidence of the commercial transaction is the XML object, and this evidence is regulated and recognized by the authorities of the countries, who give legal effects to the artifacts and trust in their use to the market when they use digital signatures that are also legally recognized. - Colombia uses ubl-Invoice and other documents, digitally signed, as securities that circulate in the market for the sale of "discounted invoices", a type of factoring, and authenticity, integrity and non-repudiation are protected by the legislation, and the availability is protected by a State entity through public storage services and an approach to the time-stamp; in the future these dematerialized documents will be registered and made available in decentralized blockchain services, and rules will be designed in smart contracts that will be based on conditions written in a formal XML document, stored in the syntax and semantics of the chosen vocabulary, and thanks To techniques such as XAdES-EPES from etsi.org, the Merkle trees of the b lockchain will be reinforced with the cryptographic summaries (sha-2, sha-3,…) used for the digital signature, and the PKI + PKC of the blockchain nodes will be able to be reinforced by means of digital certificates from CAs recognized in legal jurisdictions, well defined. - Colombia and other countries of the American continent, and the European Union process many millions of XML documents daily, with vocabularies or with their own XSD schemas.