In migrating users from an Old Instance to a New Instance I discovered the User Migration App only supports data from a few apps: User information, Forms, Files, Profile, Calendar, and Contacts. Furthermore, my Old Instance runs:
Nextcloud 25
Calendar 4.5.2
Deck 1.8.6
Tasks 0.15.0
Where as my newer instance has the following versions:
Nextcloud 27
Calendar 4.5.2
Deck 1.11.0
Tasks 0.15.0
There are inconsistencies though, newer Deck has an Export feature, but this exports a .csv file which can’t be imported anywhere.
So I figured out this hacky (non-code / CLI) way to do this data migration for my Deck Boards → Task Lists so I can preserve some data integrity.
Old Instance
Step 1.
Navigate to Tasks app and select the options for each Deck Board you want to migrate and select Export
This returns an app-generated--deck--board--ID-2023-10-21.ics file that can then be imported via the Calendars app. I think you can also export from the Calendar app, but I did this approach from Tasks app for best data integrity chance.
New Instance
Step 2.
Go to the Calendar app and click Import calendar
Once this is completed for each of your Calendars, you will see calendars of your Task Lists (Deck Boards) imported as Calendars. And if you then go to Tasks app you will see the content all imported correctly.
However, the data does not exist in Deck App despite the titles suggesting it does as those are Deck-in-name-Only Task Lists. For some reason the pipes don’t flow between all three apps (at least on NC 27 and app versions above).
Luckily, there’s a hacky way to get data back into Tasks that’s not too annoying 
and there should be some way to bring this into Deck, but I haven’t figured that out, yet… 
Step 3.
You should rename the lists to avoid any confusion of data with actual Deck Boards.
It’s a bit too bad the data seems stuck in Tasks app and not Deck but this is where I got to!
I recall previously migrating ownership of Deck Boards between users (on the same NC25 instance). I remember doing that via Tasks app and it was rather straight forward, but it seems either the ability to drag tasks between Tasks Lists (created in both Deck & Tasks) is no longer possible.