At the moment I am a little unclear whether intarray is effectively a deprecated module, or if it can still provide some distinct advantages - e.g. performance, features or just a handy syntax -- for new projects too?
A quick skim of this and the generic array functions/operators documentation shows that numerous functions and operators exist for intarray that do not have equivalents in core. For that alone I'd say this extension remains not-deprecated.
Its implied that it also performs better, ostensibly because the code need only considered non-null integer arrays whereas the general functions have to consider arrays of all types. I'm not aware of any recent benchmark runs that would indicate whether improvements to arrays in recent versions of PostgreSQL have narrowed or eliminated that gap.