What Changed in Card-to-Card Attachments and Relating vs. Linking?

[Nils Geylen]

Still a bit lost what the exact mechanics are between attaching a card to a card , linking those to the source, or mirroring. Especially in view of the non-Premium "Card Linking"...

I'm asking because I used to "relate" cards between boards--from our archive to a shared board--and observers did see some grayed-out attachment placeholder, but afaik only got a message like "you can't see this, go ask your admin".

Now it seems they get to see a button to send a join request?
With the season kicking in I'm starting to get one request after another.
I never got those; these buttons weren't there before, right?

Also, before, I seem to remember being able to "unrelate" those cards?
I now see "Linked > Remove"...

Now I'm deleting the attachment on the observer target board, and surprisingly the "preview" on the source card linking back to the former attachment remains--I'm not sure this was the case before either?

I'm sorry, but after years of incrementally improving routines, this seems drastically different behaviour, and I'd like to know the exact parameters.

I always frowned at those "Join the Workspace" suggestions that people obviously clicked. This sudden surge in requests to join a board is reminiscent of that.

Searched so far: 
Add an attachment to a card
Related cards & boards: keep your Trello life connected (the original announcement in 2017)
Card mirroring

These pages just make me doubt what I even know anymore. If someone could point me to a resource page that explains the distinctions here, I'd be grateful.

[Mike - Dreamsuite Consulting]

Hi Nils, I think I can distil this down...

Linked cards in Trello are simply URL's

So when you link a card, you are simply putting a URL to another card as an attachment, which Trello cleverly uses smartlinks to display the information from that URL.

If you add a link to Card B on Card A. A "Connect Cards" button appears, which simpliy reciprocates the link, it places a Link to Card A on Card B. I think it is beautifully simple and works really well, especially with automation.

The reason for the Access demand is because it is just a URL to a card on a board, so it relies on the person viewing it to have access to the board that card is on.  
If Card B is on Board B, when a member of Board A who doesn't have access to Board B opens card A, they will see that they don't have access to to Card B

For the simplicity of this method that Trello has adopted, I cannot see any other way to deal with cards that people don't have access to.  

I can't remember for sure (as I usually use automation to link cards) but I can see that there isn't an option to disconnect the cards, that works on both cards. I think there used to be.  

In this instance I recommend using the automation below: