TS 4.0 has some slightly breaking changes as is usual for all TS minors and majors (it does not follow SemVer).

Prettier 2 has some fairly breaking changes as will ESLint upgrades.

All of these should appear editor, linter, formatter, or build errors, and a good portion will be automatically fixable, but is nonetheless quite breaking.

Wanted to upgrade to Jest 26 in v0.14.0, but TS 3.8 was required (which I didn't also add because too many breaking changes).