interesting

If you use *BOUNDARY_SPC keyword as a constraint you can use DEATH parameter to set time when it is removed.

I suggest adding structural damping for the period before constraint removal to damp initial oscilations.

Another option would be to apply the equivalent forces and reduce them progressively to 0. Otherwise the simulation will have dynamic loading, and if I understood you want to simulate a removal, not a sudden failure.

You can terminate your simulation and perform a restart at 0.5 seconds and use the *DELETE_PART keyword.