"Unfinished business" is the term Brendon Hartley used when asked about his exit from F1 at the end of 2018. Unfinished is an appropriate word to use, the Kiwi failed to finish in seven races, a tally that only Daniel Ricciardo beat, but even when he was able to finish the results were far from sufficient.
Losing 29-4 to teammate Pierre Gasly, not many would argue that it was a tough season for Hartley. Toro Rosso exists to serve its senior team Red Bull and provide them with future drivers. They simply could not afford to use one seat on an underperforming 29 year old.
Hartley has racing pedigree, you don't win a World Endurance Championship without it, but this stint in F1 just didn't work out for him. A lovely guy but four points isn't going to cut it.