Is it possible to use ABS cement to glue together THREADED ABS union fitting on P-trap for permanent connection?
Don't ever glue a fitting that is designed for threaded pressure fit.
The threaded fittment allows the piping to be lined up then tightened for a leak free connection. Nobody but an expert could possibly line up hard glued piping under a sink. The standard fittings sealed by plastic seals, held by threaded connections, has a range of adjustment in translation and rotation about multiple axes to give leak free connections.
Sweated copper drains can be lined up because they can be adjusted to fit and then all the joints sweated at the end without disturbing the fit. This cannot be done with glue joints. AFIK it is not possible to dry fit glue joints and then squirt glue into the joint.
What often happens is the plumbing under the kitchen sink gets bashed in the course of using the undersink space for storage or for a trash can leading to a leak. Hence some people try to make solid connections, but this cannot be done without special expertise.