Thank you stopAI for that input. That brings another player previously unknown to me to the game, dex.

I have been thinking.

a) Since shed is missing the crucial feature – at this time – to halt processes at the user's logout automatically, I have decided to look for another (/temporary, I hope) solution for pipewire.
The daemon solution sounds fine – except the --user option does not work on Daedalus. .oO( Right, that's why I looked into the shed option in the first place. Huh, I guess that's the point where debdog bites his own tail. )

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b) IF shed (or any other manager for user services) wants to be a replacement for the systemd user services, it probably has to behave somewhere along these lines:

One shed service been started by the init system. One instance of shed monitoring the entire system. Which on each user login starts a subprocess for that user which then checks /etc/$somedir for "init" files on which services it has to start EXCEPT the user's $HOME/$somedir states otherwise. (This is for packaging reasons, so packages like pipewire can install their service init files into /etc/$somedir BUT in special cases the system admin can adjust that on a per user basis inside each user's $HOME/$somedir.)

On user logout the said subprocess receives a SIGTERM (or whatever) signal, then terminates the services it started before stopping itself.

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So much for theory. But if it would be that easy, EDX-0 probably would already have implemented that.

Edit: Maybe .xsessionrc has a section for commands ran on logout?

A solution that works for sddm+fluxbox/LXQt/Xfce using shed!
(This howto is intended for users testing shed with pipewire only!)

Install packages according to the first few posts of this thread.

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Install shed, use following commands to the dot if you're not able to decipher them!

cd ~/Downloads
git clone https://github.com/eylles/shed.git
cd shed
# Check for the highest release with:
git tag
# Put the highest number one in the following command, at the time of writing this was v0.2.0:
git checkout v0.2.0
make install
# Copy the scripts to a user independent location:
# (If your user is in the staff group, no "su -c" required.)
su -c "cp ../../.local/bin/shed* /usr/local/bin/"
# Remove the user dependent scripts:
make uninstall

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Create service files for shed to start the pipewire required services, on multi-user systems this has to be done for each user:

mkdir -p ~/.config/shed/services/
echo -e "NAME=pipewire\nEXEC=/usr/bin/pipewire" > ~/.config/shed/services/pipewire
echo -e "NAME=pipewire-pulse\nEXEC=/usr/bin/pipewire-pulse" > ~/.config/shed/services/pipewire-pulse
echo -e "NAME=wireplumber\nEXEC=/usr/bin/wireplumber" > ~/.config/shed/services/wireplumber

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Add the following commands to each user's ~/.xsessionrc file:

export XDG_CONFIG_HOME=$HOME/.config/
export GUI_SESSION_PID=$$
/usr/local/bin/shed &

Copy'n'paste the following command to do so:

echo -e "export XDG_CONFIG_HOME=$HOME/.config/\nexport GUI_SESSION_PID=$$\n/usr/local/bin/shed &" >> ~/.xsessionrc

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HOWEVER this only works for the first time login. shed sadly does not work as expected and does not kill the services on logout. The experiences accordingly will go the shed thread.

FWIW, I am with Steve. Of course Debian as well as Devuan are distros. Versatile ones compared to others but still distros. Plus, Steve pinned down the issues with Devuan in general and regarding to PW pretty well. He was the only one here the past few days providing solid background information on the situation at hand. And the possible options he listed on how Devuan might proceed here and related issues seem very well considered to me! RESPECT!

What kind of bothers me is, this thread – which resides under "How to:" – has a lot of unhelpful, unnecessary comments. I came here to learn about PW – How-to PW my LXQt environment – and not to read about what non PW users think about PW. Such comments make reading and comprehending a tedious task. Sharing one's opinion is fine but please, not inside "How-to"!

Thank you steve_v for bringing light to the whole situation and my totally wrong assumptions. I had no idea this would be such a rabbit hole when I started looking into PW yesterday.
It was not the first time that users on IRC asked and talked about PW. So I thought I'll try it for myself and maybe then be able to share my experience on the wiki. Now I realize, once again, I went way over my head. Funny how all this automagic userfriendlyness created just another hell.

Is there a reason why no one so far has a solution for a sysvinit script? Have there been attempts? And if so, what were the pitfalls?

You probably have to enable multiarch: https://wiki.debian.org/Multiarch/HOWTO#Usage

The wine packages shipped with Jessie are ancient, though. Preferably use the ones provided by the wine project:
https://wiki.winehq.org/Debian