I've been playing MTG for a very long time and remember when random effects and coin flip cards were made a no-go in expansions. They've clearly made a comeback with cool cards like Mirror Match and Outlaw's Merriment. What caused the change, and what are the design rules for making new cards with random effects?

They’ve never gone away, just ebbed and flowed based on R&D whim at the time.

The problem with blue's slice of the pie is that it's too loosely communicated. We can't see what blue isn't supposed to do in the same way we can other colors. Maybe it doesn't get proactive cards? Nope, Delver is OK and one of the best. Maybe it doesn't get hard removal? Nope, Mirror Match is OK and virtually does that. There should be tools blue can't touch (even in a blue way) otherwise its weakness isn't clear. This is the core reason Spine of Ish Sah is OK, but 7-mana Desert Twister isn't.

Not all Blue’s cards are reactive. Blue’s answers are reactive. Blue also has the fewest and on average the weakest creatures.

You also mention repeatedly that one of Blue's main purported weaknesses is that it can't destroy things, but Mirror Match pushes two of Blue's abilities until the card basically says "Destroy all attacking creatures." This is no different than Blasphemous Act, which you claim is a bend (so much damage that Red basically gets "Destroy all creatures"). They are cards that are problematic because they are pushed too far.

Blue doesn’t get “destroy target creature”. It does get put a blocking creature token in to play at instant speed. It can reactively interact with an attacking creature. That’s not the same as universal removal.

Re Mirror Match VS Ezuri's Predation: But isn't this Blue effectively destroying said creatures? A majority of creatures blocked in this fashion will die due to their power being greater than or equal to their toughness. The cards seem very similar: creating creatures specifically to destroy your opponent's creatures. I could see the argument if MM made 0/1's to block with, but otherwise I think I must be missing something that makes this okay for blue but not green.

Blue can already make a 5/5 creature token that can block and go away at the be if turn. Being able to kill things in combat (and there are still trucks to save them) is not the same as destroying them.

Is Mirror Match's complexity really printable at uncommon?

Probably not in a normal set but supplemental products get to bend rarity a little more.

"We could have chosen to make the new slivers some other card type but we felt we would rather keep the nostalgia of slivers rather than starting over." But you didn't really keep the nostalgia of slivers. The only the really the same between the new ones and the old ones is the name. It's like me handing you a grapefruit and calling it an orange. They look different and they play differently.

They play very similarly. They only play different in a mirror match against another sliver deck.

Does design or development ever put thought into the impact of printing powerful creatures with "X/X+1" stats? We see it in Standard with Restoration Angel and Modern with Tarmogoyf (occasionally Legacy as well) where the mirror match when both have these creatures becomes a stalemate, because whoever attacks just gets blocked and then the blocker has the advantage because the other side is tapped. With other creatures, swinging into even trades is often a smart move just to clear up the board.

Let me state for the record, that I hate */*+1. And I love */*. I’ve been trying for years to get rid of */*+1 but it’s been an uphill battle.  When I designed Tarmogoyf, he was */* not */*+1.