Hi! I'm a player in a campaign on D&D Beyond and we use the content sharing feature a lot - my DM bought a lot of books. I've recently created a new character as a backup character which has been marked as inactive by my DM. When I wanted to level up the character I noticed that I cannot access the shared content although I own an active character in the campaign. As it has been shared here I should be able to do it this way. 

Inactive characters do not get access to content sharing, this is by design. Inactive characters do not count towards any limits, so it allows a character to remain in a campaign without taking up sharing spots.

The thread you linked to was a request for that to be changed, it wasn't stated it should work the way you're expecting

 Seems that thread you linked that clearly says "request" already has a couple of answers to your question. Like this one:

Quote from DxJxC >>

Only active characters in a campaign have access to the content. Inactive characters are inactive (not being used or updated).