It had 13 followers.
As part of the routine maintenance of Area 51, proposals that have absolutely no activity for a period of 30 days are subject to deletion. That means absolutely NO activity; no new questions, no comments, no voting… absolutely nothing. This is an extremely conservative measurement. It takes only the tiniest bit of activity to keep the proposal going.
But After 10 months in development, the "Proto-Indo-European language and culture" proposal had only gained 13 followers. When activity dropped to zero, the proposal was abandoned and removed. If you have reason to believe there is renewed interest for a second go-around, feel free to relaunch the proposal.