this is sad that we cannot get KX back. There are not just a few topics, but they are plenty of them which are so important. Today I was looking for these topics, for example
1) how to get rid of divergence issues in FDTD
2) inverse design ( there was a nice discussion on inverse design at KX)
3)GDS automated export
There would be definitely more topics I would miss from KX
KX was helpful for the first-hand solution to errors, bugs, and other problems before posting a question on a forum. I hope that this point will be considered by Lumerical :)