As I understand it, version 6.5 of plasma finally supports session recall when running Wayland.
Can anyone tell me whether sessions are remembered across protocols (either automatically or by copying some files from one config directory to another), or whether the session layout has to be rebuilt from scratch on Wayland?
Why don’t you try it yourself? 
I have no idea - I don’t use the feature - but as Wayland is the primary - my guess is that you will need to recreate your session.
Login using Wayland Session - check with system settings → scroll down to session → if you activate the option When session was manually saved - then logout and relogin - see if the session load - you might be in luck - but as said - I don’t know.
Mainly because if it’s going to need rebuilding application by application and tab by tab, I’ll wait until I’ve got some free time.
(Also I’m not sure where to start looking if there is a way to do it by copying config files).
Fair enough.
My guess is that they will be .state files.
A couple of configs to check
~/.config/ksmserverrc
~/.local/state/plasmasessionrestorestaterc
Anything in the ~/.local/state might be relevant
Under Wayland the session is restored since a year or so I think.
I think there is some misunderstanding. Restoring a session under Wayland doesn’t mean the application windows’ positions would be restored. The applications/windows will be there (and tabs inside the windows too) but all the windows are cluttered on a single desktop after restoring. I think this is what you refer to somehow - and there is no change in 6.5 because Wayland doesn’t support the required functionality to position the windows as far as i know. I think it’s still in discussion.
For me personally this isn’t much of an issue since i don’t leave the session unless i have to reboot.
You can work around that by way of window rules.
It seems to have at least some. For example, when I open the laptop lid and its screen activates, Teams flips from the monitor to the position I had it on the laptop display. It also returns to its same position on the main monitor when I close the laptop display. 
Thanks to all who commented.
I managed to test a few things out on a much lighter desktop than my main workstation, from what I see:
As a Linux user of well-nigh 30 years, including both KDE and Manjaro from day 2 of their public availability, I can’t help feeling this changeover is going to be a mess of the same class as glibc2 and KDE4.
I’ll try to put in a feature request for full session restore (I think it should go to plasma-desktop, with possible escalation to Wayland).
I think this Brodie clip could be related.
I think you’d be wasting your time, because as I gather, they’re already working on that — it has been a work in progress for a long time already now.