There were several listed in the original announcement, but the main reason to acclimate your own process to them at this point is simply that the others will be going away in the near(-ish) future, 
.
(They’ve already hung on much longer than originally scheduled.)
Another option is to figure out how to deploy your own instances of the legacy builders. (These are available as an open-source repository, under the non-obvious name of RCHAB.) The neater Nix/Guix approach also has many vocal proponents here—although that way is not a small transition.
In all three scenarios, Fly.io no longer hands out freebies in the form of forgetting to bill for the huge performance vCPU class Machine, the large volume, …