I cut Mentor.
I also have been running Thieving Skydiver as a value creature against Shops.
MBT is sometimes reasonable against Shops. You can trim some of the removal because Showdown means you don't have to care as much about Leovold, Tarmogoyf and other creatures that you would otherwise struggle against.
I am currently trying to make showdown work in a jeski xerox deck and am finding 2-3 and 2 teferi to be about the right number. The biggest issue am having is between this dreadhordes, mentor, bolts, and the nessicary artifact hate I am running into low blue card density issues. Any good ideas for counterspells that can help make up for not running FON in the shops matchup and aren't completely dead otherwise?
Just so TMD has a thread for this card in case people want to talk about it.
Again, my process for evaluation isn't necessarily labeling cards "playable" or "unplayable" or trying to break Vintage by whatever definition, but exploring the amount of interactions a card has access to within the confines of the Vintage format.
Showdown of the Skalds has a number of interesting interactions like turning your cheap value creatures into de facto Mentors, pumping Dreadhorde Arcanist up to cast essentially any instant or sorcery, or even giving Blightsteel Colossus the extra oomph it needs to crash through a wall of Eldrazi for lethal (if you watched @IamActuallyLvL1 's stream last night).
It does this while dodging relevant counterplay from the opponent like Flusterstorm, Pyroblast, Leovold, Hullbreacher, and Narset. The Sagas have been viewed as an attempt to "fix" planeswalkers by limiting the effect they can have, but Showdown has an Storm-like second mode that is much more broken in Vintage than in other formats. The front half is at a good rate and there are all sorts of interesting synergies with bounce like Teferi or Chain of Vapor (LSV featured that deck but I feel if you are not running creatures, you are not maximizing the potential of the card).
I have been brewing with this card more than anything else despite how excited I was for Lurrus's unbanning, because I found it so powerful and fun.