Feature availability is usually limited by system hardware - CPU and available RAM being critical to advanced functionality such as Stage Manager. Stage Manager is technically reliant upon available RAM - this being limited to 3GB in iPad9.
The new Stage Manager feature was originally being developed for iPad models with an M1 SoC/CPU, but was later extended to include iPad models with A12X CPUs. Stage Manager is available for these models of iPad:
- iPad Air (5th generation)
- iPad Pro 11-inch (all generations)
- iPad Pro 12.9-inch (3rd generation and later)
By implication, other models of iPad do not meet the current minimum system requirements. Models currently supporting Stage Manager have 8GB or more of system RAM.
Whether or not Apple will eventually be able to extend limited Stage Manager support to older models of iPad is unknown.