From @joshlawton in #34103 (comment)

Just to confirm, this setting only accepts a boolean value and does not enable explicit font-feature-settings like ss01 or cv69? That is, even if terminal.integrated.fontLigatures is true, the font may not necessarily match the style sets and character variants enabled in the editor?

// settings.json

// Works
"terminal.integrated.fontLigatures": true

// Won't work as expected at this time?
"terminal.integrated.fontLigatures": "'calt', 'liga', 'ss19', 'cv69'"

Compare editorOptions.ts to terminalConfiguration.ts.