OSGi support was removed with d06f533bb975f41a5e86c990e1bb830b7e7dc923 in the master branch, which will become Smack 4.4, because the used OSGi gradle plugin was deprecated by gradle and the recommended alternative turned out to be not drop in replacement (at least for Smack): Simply substituting the plugin did not work.
As I have no personal or monetary incentive for OSGi, but for obvious reasons do not want to carry around a deprecated plugin, the decision was made to remove OSGi support for now. Of course that does not mean that I would oppose OSGi support in Smack. Any help is appreciated, be it a code contribution or hiring me or someone else to reestablish OSGi support in Smack.
Add the OSGi properties to the manifest to produce an OSGi deployable jar file.