Hi Matt from Maine,
it was added to improve Accessibility. And with current version the only way to remove it, is to edit template code.
The tabindex attribute on an <a> (anchor) tag is all about accessibility and keyboard navigation. It determines if and when an element gets focused when a user presses the Tab key.
Indeed, when value is not "0", "1" or "-1" WAVE tool treat that as "Warning" problem.
In that case, I fully agree we have to make deeper investigation, and correct that in next update. I shared topic with developer team.