You cannot option a player off the 40-man roster, and you cannot option a player in the off-season.
The Phils could reinstate Herrera, and then try to outright him. They'd have to get him through waivers (and given his baggage and his contract, he might clear). Yes, he'd be eligible for the Rule 5 draft - but if he clears outright waivers, he wouldn't be taken in the Rule 5 draft (which is more restrictive about what a club can do with an acquired player). And yeah, the Phils could then just let him sit in AAA for two years, and pay him his $20.2 million.
If they really don't want him in consideration for a 2020 active roster spot, asking outright waivers would get him off the roster - but if he has any trade value, this move would abandon any potential trade return - because outright waivers are irrevocable, so if they ask waivers and he's claimed, they get nothing back.
Whatever the options, we'll likely see this story play out in the beginning of November, after the World Series, when the Phils have to finalize their winter roster.