Official development for the Project DIVA series has largely been discontinued by SEGA in favor of Hatsune Miku: Colorful Stage! . However, the community continues to release "new" content:

While the "free download" tag is a popular search term, it’s important to clarify that Hatsune Miku: Project DIVA Mega Mix+

The safest way to get a "free download" is the official trial available on consoles. Nintendo Switch : A free demo is available on the Nintendo eShop

While purchasing the game is the most straightforward way to get started, there are some legitimate methods to obtain Hatsune Miku Project Diva Mega Mix for free: