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Is the J-Link library for the development of Creo Parametrics extensions still working and supported?
If it shows up as an option when you're installing Creo, one can only assume it is still working. I think a lot of add-ons for Creo use this capability, so I doubt it would been discarded by PTC.
Try searching on the internet to find instructions on how to use it, etc.
Yes, it is still working and under maintenance/development and is a.k.a. Object TOOLKIT Java or otk_java.
See [Knowledge Hub] Creo Parametric Object TOOLKIT Java Free https://www.ptc.com/en/support/article/CS320898