I am well aware the WD100 has been out of support for some time and therefore only works with the firmware of the period.
Nonetheless, I would be very interested to add one to my system even if it means downgrading the firmware and/or app.
According to the community post below it was still possible in 2018:
Has anyone had any success in downgrading the firmware to allow WD100 compatibility and does anyone have links to the required firmware?
You can’t downgrade Sonos firmware and the iPod doc will not work on current S1 or S2.
You can however j use the line in on the latest Sonos speakers and connect an iPod via that. What are you trying to achieve?
I have kept mine just because it is a nifty piece of engineering I hate to bin. For those that have stayed back in time to keep it running, this use may still be unique: set it to autoplay with a Sonos speaker and you can have music play from a docked device with downloaded music, on said Sonos speaker in places where there is no WiFi. You do need mains power of course.
Thanks
FYI, I didn’t mean to mark your reply as an answer! 
♂️
At home, once you move all your purchased music to a NAS, you can get your music to play for free, no subscription. There is something else happening in the world of Sonos that will kneecap this pleasure in future, but that is a different subject.
At home, the dock served as a neat way for any visitor to slot in their iPod, and play music wirelessly via your Sonos set up. That is the only functionality that is missing now in Sonos after the demise of the dock.
I let my dock become a museum piece and was very upset about having to do that back in the day; now, not so much.
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