With the trunk working again, it's less critically important, but I do have one
reservation in hindsight about TIP 562, which I understood in simple terms as
dropping support for channel type versions 1 through 4, but retaining support
for version 5 channels.

That's not really what the TIP did, though. Some existing version 5 channels
define a closeProc and may expect it to be called, but after TIP 562, it will
not be called any more.  We really created a version 6 of the channelType
but failed to bump the number.  I think I'd be happier if we made that bump
to TCL_CHANNEL_VERSION_6 as part of the post-vote implementation review.
We would make version 6 available in 8.7, and allow only it to go forward to Tcl 9.

That's what we're already really doing; this would just label it more accurately.