Mana Vapors 
Sorcery
$0.33 #22219 1.51 26.3 years
Lands target player controls don't untap during their next untap step.
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Cards similar to Mana Vapors 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.
Exhaustion90%
$0.20↓#17523↑1.33↓29.3 years↑
"Creatures and lands target opponent controls don't untap during their next untap step."
Reduce // Rubble86%
$0.16↓#239380.31↓9.4 years↓
"Up to three target lands don't untap during their controller's next untap step."
Stensia Innkeeper86%
$0.13↓#29131↓0.69↓10.1 years↓
"When this creature enters, tap target land an opponent controls. That land doesn't untap during its controller's next untap step."
Vorinclex, Voice of Hunger82%
$4.93↑#1758↑2.61↑15.3 years↓
"Whenever an opponent taps a land for mana, that land doesn't untap during its controller's next untap step."
Chandra's Revolution80%
$0.07↓#216760.53↓9.6 years↓
"this card deals 4 damage to target creature. Tap target land. That land doesn't untap during its controller's next untap step."
Mana Skimmer77%
$0.10↓#29885↓0.43↓19.9 years↓
"Whenever this creature deals damage to a player, tap target land that player controls. That land doesn't untap during its controller's next untap step."
Winter's Night74%
$0.87↑#29674↓0.15↓30.2 years↑
"Whenever a player taps a snow land for mana, that player adds one mana of any type that land produced. That land doesn't untap during its controller's next untap step."