Technically, Eli Manning is retiring. Technically, Eli Manning remains under contract until the new league year begins in March. Technically, the Giants won’t be permitted to place Manning on the reserve/retired list.
According to the league office, Manning’s expiring contract prevents the Giants from handling his retirement this way. And that’s an important point, in the event (unlikely as it may be) that Eli chooses to unretire in 2020.
Manning most likely won’t be coming back. But what if a high-end contender loses a quarterback to injury in November or December and the decision is made not to go to the next man up but to plug in Eli? If he were on the Giants’ reserve/retired list (and if the trade deadline had passed), he would be exposed to waivers before he could sign with the team that wants to sign him. And any other team could block the move by claiming him.
If the team that wanted/needed Eli were the Giants, the same hard deadline (last business day before Week 13) that applied to Rob Gronkowski’s potential return to the Patriots would apply to Eli, if he were on the reserve/retired list.
As it stands, Eli would be Marshawn Lynch, not Gronk. Eli can sign with anyone, at any time. If Eli wants to play for the Giants or anyone else at any point in 2020 -- up to and including a one-game stint for the Super Bowl in the event a starting quarterback gets injured during the game that clinches a conference championship -- he’ll be able to do it.