What
React DevTools code has been significantly reduced in the bundle.
Details: References to React DevTools dropped from 45 to 2 occurrences, removing unused debugging infrastructure.
Evidence
Removed variable b6B at line 173391 in v2.1.9 (contained __REACT_DEVTOOLS_COMPONENT_FILTERS__ setup)
Strings lifted out of the shipped bundle, so the claim above can be checked against them.