GremlinExecutor.promoteBindings was mostly an internally used method for turning bindings returned from an init script into "global" ones that would be reused going forward. This method was replaced by allowing the user to simply return a Map from the script of the "globals" they wanted to preserve. That provided more flexibility.
Removal of this deprecated method ultimately affects Gremlin Server scripts which will not preserve "globals" previously defined in the 3.0.x fashion. Users will need to upgrade their scripts to use the new Map style.