Inventor uses Excel in many different workflows. The predominant complaint & the primary reason for setting that environment variable was to improve the performance of creating holes, (which by default was using Excel to read the xls file containing all of the various information).
Setting the variable told Inventor as a whole not to use Excel but to use the LibXL libraries instead which are more performant but not as functional as Excel. This works fine for read only based operations such as creating holes but not for write based operations such ass editing iparts/iassemblies using Excel.
The work done in 2018.1.1 was to always use libXL for the hole based functionality regardless of whether the variable was set or not. So if the the variable was being set to improve the performance of the hole dialog then it is no longer required.
If however, you were setting it to improve the performance of iparts/iassemblies then it may still be needed. Note that by setting it you may not be able to properly edit iparts/iassembly tables using excel anymore. Other functions such as animating pos reps in Studio may not be fully functional with the variable set.
The work to completely remove the dependency on Excel across all of Inventor is a large project that is being approached in the order of command usage in order to maximize ROI.
Hope this clarifies,
Chris

Chris Mitchell
PDMS Customer Engagment Team
Autodesk, Inc.