The 27-year-old is being replaced by the first ever Chinese F1 driver Guanyu Zhou, with teammate Kimi Raikkonen also departing for Valtteri Bottas as the former world champion retires from the sport.
The changes at Alfa Romeo mean Giovinazzi will not have a seat on the grid for next year as all the teams have now announced their drivers for the new season.
And world title contenders Hamilton and Verstappen have already spoken to him following the decision.
Speaking as a guest on RadioRai’s ‘Non è un paese per giovani’ programme, Giovinazzi said: “Both Hamilton and Verstappen have written me a nice message and they are things you appreciate because it means they follow you.
“I raced with Max in 2015 in Formula 3 and so I already knew him very well, and so they are gestures you appreciate very much.
“But not only from them, also [Pierre] Gasly, [Esteban] Ocon and [Sergio] Perez.”
Giovinazzi has picked up just one point so far this season with just two races remaining. Comparatively, Raikkonen has accumulated 10 points.
The drivers travel to Jeddah for the Saudi Arabia Grand Prix this weekend, before the 2021 campaign concludes in Abu Dhabi on 12 December.