In discussions about this topic over the weekend with other SFWA members at Nebula con, I was led to believe that the supporting membership was only 5-6 years old.I don't know how this sort of thing gets started, but it's clearly being repeated by people who don't actually know a whole lot of about how Worldcons run and how WSFS works.
Supporting Members of the current Worldcon have always been able to nominate. I'm not exactly certain when Worldcons started selling Supporting memberships, but it certainly goes back into the 1960s, and the oldest versions of the WSFS Constitution that I have say that all "society members" (a phrase that was understood at the time to include Supporting members) can nominate. Allowing supporting members of the current Worldcon to nominate is not a recent innovation. It goes back at least fifty years.
Members of the previous Worldcon have been able to nominate since 1989. An amendment to the WSFS Constitution ratified at the 1988 WSFS Business Meeting extended nominating privileges to members (attending and supporting) of the previous Worldcon, effective as of the 1989 Hugo Awards. Allowing members of the previous Worldcon to nominate is not a recent innovation. It has been happening for nearly thirty years.
Members of the following Worldcon have been able to nominate since 2012. An amendment to the WSFS Constitution ratified at the 2011 WSFS Business Meeting (Item 3.4, pages 20 and 40-41) extended nominating privileges to members (attending and supporting) of the following Worldcon, effective as of the 2013 Worldcon. Allowing members of the following Worldcon to nominate is a recent innovation. This year is only the fourth year it has happened.
Note that everywhere above where I say "supporting and attending members" or variations of it, that also includes any other class of member (e.g. Young Adult, Military Discount) granted WSFS membership rights by the individual Worldcon. The WSFS Constitution gives Worldcons a great deal of leeway about granting WSFS membership rights, other than they can't sell a membership with the rights of a Supporting membership for less than the cost of a Supporting membership. (Section 1.5.8 of the current WSFS Constitution, effective this year, adds this restriction.)
While there has been one change that was relatively recent (extending eligibility to the following year's members), it seems unlikely that this had much impact on the results of the nominations in the past two years. In any event, supporting members have been able to participate for as long as I've been involved with Worldcon (1984), and in fact have been participating for over fifty years. Proposals to strip them of their voting rights, particularly if they were members of the current Worldcon, would be a significant and drastic change in Worldcon practice.