I changed my previous plugin version to have:
<idea-version since-build="8000" until-build="10372"/>
in the plugin.xml file since I found that I had to recompile my plugin to work with #10666. However, now IJ8.1.3 says this plugin with this modified plugin.xml is incompatible. Why? 8.1.3 is #9886 which is between the two version numbers I specified in the plugin.xml.