Now let me get more specific performance data by analyzing which archetypes are playing Thoughtseize and their winrates:
Overall Presence: Thoughtseize is the 9th most-played card in Pioneer maindecks with 24.71% format presence (254 decks playing an average of 3.96 copies).
Golgari Midrange (90 entries)
Black Demons (41 entries)
Abzan Greasefang (31 entries)
Rona Combo (9 entries)
Other Notable: Orzhov Midrange (21 entries, 48.24%), Rakdos Midrange (10 entries, 48.72%), and various other BGx shells.
Thoughtseize performs best in Golgari Midrange, which is both the most popular home for the card and maintains a positive winrate. The card's versatility makes it a format staple across multiple black-based midrange and control strategies, appearing in nearly 1 in 4 decks.
Sources: Data from 37 tournaments (81.1% MTGO, 18.9% Melee). Recent events: Pioneer Challenge 32 (Feb 1), Pioneer Challenge 32 (Feb 1), Pioneer Challenge 32 (Jan 31)