Grixis Delver has been a minor player in the Modern metagame dating back to its introduction around the 2015 Magic Online Championship Series. As an archetype, Delver has been a combo deck killer in several formats. Quick, efficient threats that hit hard backed up by permission spells force you opponent to act in uncomfortable and clunky ways.
As I've talked about before, Modern is moving towards being more creature-heavy. We have not seen as much Grixis Delver as in the past, but Thought-Knot Seer, Grim Flayer, Arcbound Ravager, and Death Shadow have become quite common. Even still, like most established decks in Modern, Grixis Delver can Take Down a tournament with some tight play and clever metagame adaptations. For instance, Kevin Jones won the last USA World Magic Cup Qualifier piloting Grixis Delver.
Did I mention adapting your deck to win a Modern tournament? Kevin utilized Young Pyromancer in his Grixis list, which have been sidelined in lists for quite some time now. I think that this could be a solid move, since Young Pyromancer is a "broken" card to increase the power level of a fair deck and is likely strong in creature matchups. Kevin also played perhaps the smallest amount of countermagic I have seen in Grixis, opting for a heavy dose of creatures and removal, something that certainly worked out in this case.
I think it's fair to mention that in the past, I've disliked the Young Pyromancer package. One thing to do when testing a deck is to try exact or near exact copies of a well-performing list in order to see what works and what doesn't before making any changes. Young Pyromancer certainly has potential, but I still think a traditional version that is more controlling is best, which is certainly my preferred play style.
Jones also included a relative newcomer in Collective Brutality. It's only a one-of, but preparation through tech choices and adaptations can win you that extra match to place you in the Top 8. In the past Grixis Delver would only include Thoughtseize, and even then only in the sideboard, and personally I never found it good. Fundamentally, hand disruption is bad in Delver of Secrets-based decks. Why? Delver decks play to win the board through efficient creatures, putting yourself ahead in order to tempo your opponent out of the game. With that established, it looks to win "the stack" with permission,or the board with removal. Delver and Delve creatures help you get ahead so your spells should follow suit (Mana Leak, Bolt, Daze, etc.).
Thoughtseize costs you mana, and while you get to take a strong card, your opponent still gets to use their mana on a different card to work against the goals you have set up with the rest of your deck. You would rather have them spend four Mana casting a Thought-Knot or Restoration Angel only to get it Mana Leaked or Terminated than you would Thoughtseize it away from being cast in the first place and allowing them to resolve a different spell. I think Brutality is a clean fit because of its flexibility — it can play to the board, give you additional reach, or act as disruption.
With more Counterspells and additional Kolaghan's Command, the deck is now are very flexible. Grixis Delver is good at choosing different roles within certain games and matchups. With more reactive spells and without Gitaxian Probe costing life you have a greater ability to assume a controlling role, even in matchups where you are typically the aggressor, which is what I enjoy most about the deck. You have so much flexibility to attack a small window of time and win games, but also to step back and use your card advantage engines such as Snacpcaster Mage and Kolaghan's Command to bury your opponents in a control game. That is why I would likely move away from Pyromancer, freeing up slots for the more traditional Remands and Commands. That said, this is my take and to each their own, and I would advocate trying all versions before drawing any conclusions and settling on a list for your next tournament.