This following portion of the documentation is incorrect, strictNullChecks does not have to do with the "not all code paths return a value" check, it is noImplicitReturns that checks this.

If using strictNullChecks and doing exhaustive checks, TypeScript might complain "not all code paths return a value". You can silence that by simply returning the _exhaustiveCheck variable (of type never).
https://basarat.gitbook.io/typescript/type-system/discriminated-unions#strictnullchecks

Playground example:

• strictNullChecks is off: https://www.typescriptlang.org/play/?strictNullChecks=false#code/JYOwLgpgTgZghgYwgAgMoEcCucooN4BQyxyA1qACYBcyARAM5Y4S0DcRJ9wAXhDSJgC2AI2jsAvgQKhIsRCgBKEBGDggA5gBt8HYuRDU6uFWq0t2JZAHdgFMAAt+Q0VAsl7EYOvtgnIsQSS0uDQ8EjIAMLAUAjayISW+oa0CNGx5rrIUHAUwJj0fi4SUmAAngAOKKj2cJXIALxoTLjIAD7ISiYace1RMdrsBDCYICrAAPYgyMxwABQFaDWVAJTxmfQ2YAj2yPMAdEmrCZYkCHD0KAzNLDS4YJhQU-R7XLzIAFTIz68QbifIZwuRmUqm6NyyEHujy+exsdh2n2eHi8Pj+J0Bl1S-XBdweUwAsnAHHsAAoASQ+MOyuXylOe1Ly9DRlgoEHgmE0vky-wQk3oYGQAH0IAAPGr5MDAABuEAiHgQpH4EBlUAaXzRknEQA
  • Not all code paths return a value.
• strictNullChecks is on: https://www.typescriptlang.org/play/?#code/JYOwLgpgTgZghgYwgAgMoEcCucooN4BQyxyA1qACYBcyARAM5Y4S0DcRJ9wAXhDSJgC2AI2jsAvgQKhIsRCgBKEBGDggA5gBt8HYuRDU6uFWq0t2JZAHdgFMAAt+Q0VAsl7EYOvtgnIsQSS0uDQ8EjIAMLAUAjayISW+oa0CNGx5rrIUHAUwJj0fi4SUmAAngAOKKj2cJXIALxoTLjIAD7ISiYace1RMdrsBDCYICrAAPYgyMxwABQFaDWVAJTxmfQ2YAj2yPMAdEmrCZYkCHD0KAzNLDS4YJhQU-R7XLzIAFTIz68QbifIZwuRmUqm6NyyEHujy+exsdh2n2eHi8Pj+J0Bl1S-XBdweUwAsnAHHsAAoASQ+MOyuXylOe1Ly9DRlgoEHgmE0vky-wQk3oYGQAH0IAAPGr5MDAABuEAiHgQpH4EBlUAaXzRknEQA
  • Not all code paths return a value.
• noImplicitReturns is off: https://www.typescriptlang.org/play/?noImplicitReturns=false#code/JYOwLgpgTgZghgYwgAgMoEcCucooN4BQyxyA1qACYBcyARAM5Y4S0DcRJ9wAXhDSJgC2AI2jsAvgQKhIsRCgBKEBGDggA5gBt8HYuRDU6uFWq0t2JZAHdgFMAAt+Q0VAsl7EYOvtgnIsQSS0uDQ8EjIAMLAUAjayISW+oa0CNGx5rrIUHAUwJj0fi4SUmAAngAOKKj2cJXIALxoTLjIAD7ISiYace1RMdrsBDCYICrAAPYgyMxwABQFaDWVAJTxmfQ2YAj2yPMAdEmrCZYkCHD0KAzNLDS4YJhQU-R7XLzIAFTIz68QbifIZwuRmUqm6NyyEHujy+exsdh2n2eHi8Pj+J0Bl1S-XBdweUwAsnAHHsAAoASQ+MOyuXylOe1Ly9DRlgoEHgmE0vky-wQk3oYGQAH0IAAPGr5MDAABuEAiHgQpH4EBlUAaXzRknEQA
  • No errors!