The new Umbral Shards resource is a new way to gain powerful items. It is a bit grindy though, so developers have found a compromise with players, in a way that benefits their vision and the players' satisfaction.
The recent Umbral Shards changes have not been received well by the community as a whole. Umbral SHards is a new end-game resource, for an end-game that doesn't really exist all that much now, and is something players don't feel like they needed or asked for. However, with their response to the player feedback, developers have shown that they are at least trying to work with the community and listening to it:
"Thanks for all the feedback so far on the Umbral system. One piece of feedback that is fairly unanimous and I think makes sense is limiting the ability to upgrade to GS 600 is pretty onerous. We wanted to keep the value of legendry 600s really high. But I can totally see how that is limiting and can be disheartening when you get a lot of bad RNG and haven’t gotten your Legendary roll.
Therefore, we are looking into reducing the required minimum GS to upgrade to 590. You will still need to get to expertise 600 to unlock upgrading via umbral. But once it's unlocked you could now upgrade from 590 to 600 at a moderate umbral cost. That allows those really good 590-599 rolls, to become great weapons, with a little extra Umbral.
We will try to get this change in as soon as possible."
At 600 these items will not gain an additional perk. Devs think that would potentially devalue legendary items and the chase for them too much. Once players level up mutators or their Expertise, they will be able to earn a fair amount of Umbral Shards quickly. So they could potentially upgrade a lot of 590+ items in the hunt for a good perk roll. The goal is not for the Umbral system to be a source for “new” gear, but rather a path to upgrade gear.