[RPG] What are the implications of being able to see into the Ethereal Plane in Curse of Strahd

The object is as real as it gets. It would be the same as creating any other object that can be seen and touched. The idea is to add some tangibility to your illusions via an additional aspect of magic (shadow magic). True Sight would reveal the illusion as false, but the object actually exists in space and so the creature sees that as normal. Whatever information the creature knows about the object is up to circumstances and your DM.

For instance, if the creature walked into an illusory house you made and you also made a chest real inside of it, there's no real way for it to know the chest isn't permanent (isn't real) unless he inspects it, or saw you create it, or otherwise reasoned its genuineness.

The fact that the object can deal no damage may or may not affect the way your DM rules the reality of the object. Surely a dagger made real should deal damage, and yet it cannot. My out-of-game reasoning for this is more about balance. I don't think the rule makers want you to use 1st-level spell slots to create grenades and daggers and other harmful things. In-game, i might suspect the material is weak or the object governed by special shadow magic rules. Your DM can handle the in-universe reasons for why a real object wouldn't be able to damage anyone.

So how do you handle a real weapon that can't deal damage with true sight? I think there are a number of explanations you could have for why a real dagger wouldn't deal damage, but at the end of the day, all that matters is that True Sight sees the object as real, because it's not an illusion. See is the key word here, since true sight has nothing to do with inspection, touch, or investigation. True Sight is set apart from intelligence. Think of it like a plastic bowl of fruit. It's real, right? But, until you actually inspect the fruit, you don't recognize it as a fake real item. Real being opposed to illusory here, not opposed to genuineness.

That's a 20th level epic boon (so very powerful)

The power of a permanent truesight effect would be situational. If you measure 'power' by raw combat ability, then unless you're fighting invisible creatures truesight has little effect. Even if this player is fighting invisible monsters, the rest of the party is still vulnerable. So it shouldn't break combat balance. Outside combat, truesight would also be of little use if you don't encounter illusions or shapechangers or ethereal creatures, although if you do encounter those things it solves those challenges trivially, which may be an issue for the DM trying to make interesting challenges. The range would also be a big factor. Truesight to a range of 10ft (for instance) requires the player to be standing very close to the item of interest and wouldn't be as powerful as a range of 60ft, which would reveal an entire room.

But if we want to see how powerful the ability of permanent truesight is in general, we can look at where the game designers have made truesight available for player characters. You have already noticed truesight's availability as a 6th level spell, which automatically puts it in a rather high echelon of power. But where would permanent truesight fit into the picture?

The answer is found in page 232 of the Dungeon Master's Guide, under Epic Boons. Epic Boons are only available to player characters who are at level 20 and are made to extend the power of already-powerful 20th level characters. Of these epic boons, there is one which provides exactly the effect you are considering:

Boon of Truesight

You have truesight out to a range of 60 feet.

If the game designers thought that permanent truesight (to 60ft) is something worthy for only a 20th level character, you should be very cautious with giving out a similar ability to a lower level player character. Putting a much shorter range and/or limited uses per day might (might) help balance it for lower level characters, although it will still be very powerful in certain situations.