The Legacy Pack is free for owners of Hitman (2016) on the platform they previously owned it on. It contains all of the Hitman (2016) locations (and bonus missions). It does not contain the GOTY content (patient zero, suits, escalations w/ unlocks). The (basic) Legacy Pack can not be purchased, it’s only available to people who have the basic Hitman (2016) season 1 content.
The GOTY Legacy Pack is free to owners of Hitman GOTY Bundle (2016). To get this free you don’t just need to own Hitman (2016) season 1 you need to own the actual GOTY Edition of the game. The GOTY Legacy Pack is all of The Legacy Pack (listed above) plus the additional GOTY content (patient zero campaign, suits, escalation missions with weapon unlocks)
The Legacy Pack Upgrade is for people who only own the (free) Legacy Pack and want to convert it into the GOTY Legacy Pack.
If you never had the (basic) Legacy Pack you can not buy just the upgrade, that would be pointless and they don’t let you buy the basic Legacy Pack. What’s available to you for purchase is the GOTY Legacy Pack (which is both the Legacy Pack and the upgrade together)
However, GOTY Legacy Pack purchase requires you own the actual game Hitman 2, that’s standard edition (or silver or gold), you can not purchase DLC if all you own is a starter edition (starter editions are: The Starter Pack, The Miami Pack or the Santa Fortuna Pack)
If all you own is a starter edition you need to first buy at least Standard edition then the GOTY Legacy Pack is buyable