This message is displayed on all upcoming changes when they reach Stable status and we recently changed this one and some others to Stable.
However per Introducing Upcoming Changes, a system to manage new features and changes to existing functionality in your community, not all upcoming changes are intended to become permanent in a sense that we remove the setting completely at the end and any logic in the code behind it.
In this case, Stable means we’ve turned it on for most sites, but they can still opt-out and we don’t intend to force it on as the only option.
I’ll need to make a way to indicate this for certain upcoming changes, what we expect the “end state” for it to be. For now you can just ignore the warning on the upcoming change UI, we won’t move “enable auto grid images” to Permanent status, and we won’t remove the setting.