Hi Nicola,
Thanks for using Syncfusion Products.
We were able to reproduce the reported issue. TargetID property was not deprecated in our list box control. You can continue to use TargetID property. This issue does not produce any problems. So, we have internally log the issue “Remove deprecation warning error on TargetID”. This issue will be fixed and included in our main release Vol 4, 2017, which is expected to be rolled out by November 2017.
Regards,
Buvana S