Per Deepflame in discord:
https://discordapp.com/channels/125011928711036928/360712420240523264/699222772597129238

The idea was for them to become items you would insert into the doorframe
So you could upgrade doors to different materials
And damaging doors would just break the item, rather than the whole doorframe
There's, I think, no programming lets to do to enable doors to work like that
Just needs an inventory component with owner limited access, and proper xml definitions set up
The facade system was introduced for that purpose too
So you could make "metal door" as an item, and it would fit into each doorframe, finding the right shape
So you wouldn't have to make "round wooden door", "square wooden door", "differently rounded wooden door"

You need to do the following steps:

1. On the door frame block, add an inventory component with a constraint to only accept door items.
2. On the door frame block, add the visual inventory component with matching facades to assign door frames to the right attachment point etc.
3. Make door items durable items.

Also we never resolved what would happen during construction, and importing pre-0.7 grids and saves
All castles would be without doors