In a company I used to work for, "sustaining engineering" was the team of developers that handled all of the bugs and issues reported by customers on old-but-still-technically-supported versions of the products. (The ones who worked on current versions of the products where just "engineering.")
So basically heroku will fix whatever is broken, but don't expect any new features or development.
From a business perspective, this means they will not be investing in innovation on the platform anymore. Instead, they will focus their efforts on maintaining the current operations and keeping the lights on.
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.