Well, I don't know if these items are available to non-kickstarter-backers, but I for my part think they should be exclusive to those who believed in the project back then.

Despite that, I'm still disappointet that the items have a durability and break (yes, also the Elite Grappling Hook)...

Yea Kickstarter backer items are exclusive for sure. This was years ago but I remember in a Twitch stream discussing ways for players to unlock similar versions of this content as some people simply didn't even know about the KS campaign or maybe didn't have the funds back then to support at those tiers, etc. To me it seemed like a win-win because KS backers could also work to unlock those other variations. So we were working on a solution to that but unfortunately had to cut/delete all of it as we kept running into issues leading up to release. Thankfully we were able to squeeze in the Knights armor and some other secret items.

As for the Elite Grappling Hook having durability you can turn off durability in the game settings by playing Story mode. It was a tough call with the Elite grappling hook as pretty much all other KS content was outfit/pet based so we didn't have to deal with durability.

Hope that helps!

I'm one of the people who contributed to Kickstarter many years ago. But how do I see if I received the extra game content. Well, I'm just now starting to explore the game.

I found it, it was through the humble bundle. Everything's solved.