WCAG 2.1 says to have a minimum contrast ratio of 4.5:1, but blue primary buttons of Twitter, Figma and Outlook have white text which fails to cross the threshold ratio. Is there another scientific angle to this? 
[Update] Even Google images has that color now (with a ratio of 2.55).
Accessibility modes are likely the reason. Twitter has a high-contrast mode for users who need it:
The accessibility setting of the web app now has a new “increase color contrast” toggle button. The button once turned on, activates the high contrast colors for UI elements. The high contrast mode makes it easier for people with visual impairments to use the Twitter Web app.
Outlook's desktop app adopts the operating system's higher-contrast settings when activated.
Outlook - regular contrast mode
Outlook - increase contrast mode
Unsure why Figma has a lower-contrast button - it doesn't seem to be modal.
