Another word for agnostic when used to suggest independence through a lack of concern
Solution 1:
A term synonymous with platform-agnostic is platform-neutral.
Perhaps the word you're thinking of is neutral.
Solution 2:
You might also try:
independent : not dependent; not depending or contingent upon something else for existence, operation, etc. .
The abstraction layer is OS-independent.
Solution 3:
apathetic? indifferent? blind?