Use it for RelationSyncEntry->streamed_txns, which is currently using an integer list.
The API support is not complete, not because it is hard to write but because it's unclear that it's worth the code space, there being so little use of XID lists.
f10a025cfe Implement List support for TransactionId
src/backend/nodes/list.c | 42 ++++++++++++++++++++++++++++-
src/backend/nodes/outfuncs.c | 4 +++
src/backend/replication/pgoutput/pgoutput.c | 14 +++-------
src/include/nodes/nodes.h | 1 +
src/include/nodes/pg_list.h | 21 +++++++++++++++
5 files changed, 70 insertions(+), 12 deletions(-)