Sustaining is used in Engineering to mean that it's now post-GA and there is no further development. The platform is not End of Life but there are no more features planned.

They meant what they wrote. Merriam-Webster's definition: "to support the weight of"

It means they're transitioning to the absolute minimum to keep it alive and nothing more. That could, in worst cases, mean firing everyone except one guy, or using AI to keep it alive.

Sustaining as in sustaining their shareholders.