`dump-dir` and `dump` options were remove somewhere in version `7.x` https://github.com/drush-ops/drush/commit/df3980062a89436a26db20097363da69616cbb9c

You can still find it in `6.x` branch but not in `8.x`
https://github.com/drush-ops/drush/blob/6.x/examples/example.aliases.drushrc.php

In issue https://github.com/drush-ops/drush/issues/1495 you can read:
> markcarver: What happened to drush_sql_dump_file()?

> weitzman: The feature is gone. sql-sync does not keep around transferred dump
> files any more.