Diffusion Sliver 
Creature — Sliver
$1.74 #4335 1.11 12.3 years
Whenever a Sliver creature you control becomes the target of a spell or ability an opponent controls, counter that spell or ability unless its controller pays 
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Cards similar to Diffusion Sliver expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
Parnesse, the Subtle Brush79%
$0.38↓#16974↓0.40↓4.5 years↓
"Whenever you or a permanent you control becomes the target of a spell or ability an opponent controls, counter that spell or ability unless that player pays 4 life."
Amulet of Safekeeping78%
$0.41↓#17077↓0.35↓8.3 years↓
"Whenever you become the target of a spell or ability an opponent controls, counter that spell or ability unless its controller pays ."
."
Unsettled Mariner78%
$3.10↑#39650.63↓7.4 years↓
"Whenever you or a permanent you control becomes the target of a spell or ability an opponent controls, counter that spell or ability unless its controller pays ."
."
Frost Titan76%
$0.45↓#9965↓0.19↓16.3 years↑
"Whenever this creature becomes the target of a spell or ability an opponent controls, counter that spell or ability unless its controller pays ."
."
Reality Smasher73%
$0.76↓#10679↓0.16↓10.7 years↓
"Whenever this creature becomes the target of a spell an opponent controls, counter that spell unless its controller discards a card."
Opaline Sliver72%
$2.50↑#10884↓0.94↓20.0 years↑
"All Slivers have "Whenever this permanent becomes the target of a spell an opponent controls, you may draw a card.""
Reidane, God of the Worthy // Valkmira, Protector's Shield71%
$0.41↓#6442↓0.53↓5.7 years↓
"Whenever you or another permanent you control becomes the target of a spell or ability an opponent controls, counter that spell or ability unless its controller pays ."
."
Leeching Sliver70%
$0.56↓#11172↓0.75↓12.3 years
"Whenever a Sliver you control attacks, defending player loses 1 life."