Since Feb 2009, it's announced that both 1.6.x and 1.7.x won't have more releases, hence they became unsupported.
1) Change in the tracker, current message for current versions in those branches to (dates are a proposal):
There will not be another 1.6.x release, but critical bugs may still be backported to the branch until July 31th, 2009
There will not be another 1.7.x release, but critical bugs may still be backported to the branch until October 31th, 2009
2) Then, the day after those dates, release definitively those "+" versions, so they will become "closed".
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Finally, perhaps we should also start thinking about the date to end support for 1.8 and, once agreed, publish it. Perhaps when 2.0 became stable (beta or release) could be a good moment. But that needs to be announced before. Or perhaps we want to continue with the "3 supported versions" schema. To decide, anyway.