I'm experiencing this problem on Debian Trixie and hope it can be addressed in a point release.

I stumbled on this situation trying to see how gvfs's WSD support would work. (I think gvfs-backends has a missing dependency because it wants to invoke some version of wsdd for help, and its Recommends: wsdd is no longer satisfiable; I'll investigate that on my own.) I thought I'd try the obvious thing and install the wsdd2 package, but at installation time it fails to start the service:

Setting up wsdd2 (1.8.7+dfsg-1.2) ...
Created symlink '/etc/systemd/system/multi-user.target.wants/wsdd2.service' → '/usr/lib/systemd/system/wsdd2.service'.
Failed to start wsdd2.service: Unit smbd.service not found.
Could not execute systemctl:  at /usr/bin/deb-systemd-invoke line 148.

This is not just a fluke: wsdd2.service indeed has
[Unit]
Description=WSD/LLMNR Discovery/Name Service Daemon
BindsTo=smbd.service
After=smbd.service
PartOf=smbd.service

but there is indeed no package providing smbd.service installed yet, nor was it ever suggested. The ksmbd-tools package that was pulled in indeed has a ksmbd.service unit in it, but that's not the same and it's not obvious whether that service should be sufficient (I am unfamiliar with Samba stuff). From https://packages.debian.org/search?suite=trixie&mode=exactfilename&searchon=contents&keywords=smbd.service one finds the only package providing smbd.service is the full-blown 'samba' package.

So why isn't this pulled in? It's not obvious that this would be the right thing to do; the samba package's description says
 $ dpkg-query -f '${Depends}\n' -W wsdd2
 ksmbd-tools | samba, libc6 (>= 2.38)

Because I had neither ksmbd-tools nor samba installed, APT chose the seemingly-preferred ksmbd-tools, which is very unfortunate in this instance as the samba package seems to be needed in any case. Thus I consider this a release-critical issue: wsdd2 seems intended to serve clients as a way to discover and connect to Web Services on Devices. The intended users are unlikely to have the samba package already installed, but this is the only way the systemd unit will be permitted to start.
If I simulate installing the samba package, then ksmbd-tools is removed as the latter has an explicit Conflicts on the former. This hints that they have overlapping functionality and ksmbd-tools (and ksmbd.service) may be adequate. Research is needed to see if wsdd2's unit file can be adapted to permit ksmbd.service without loss of functionality.