Why not default the display to 2.5 and 3.0 extensions only and allow people to optionally display 1.5 extensions with a checkbox. The checkbox could even provide a disclaimer that all 1.5 extensions will no longer be supported by Joomla or the extension developer.
I think there is probably a conflict of interests here between the end users and the extension developers and Joomla team. Joomla \ extension Developers no longer wish to support 1.5 as it's an extra headache supporting it with 2.5 and 3.0, this is completely understandable.
However there are end users who, for whatever reason, are not in a position to upgrade yet. If somebody has a fully operational 1.5 site that has no benefit of upgrading and they want to install a small extension to show a map or something then why should they have this choice? As long as they have the understanding that if the extension doesn't work there will be no support there shouldn't be a problem.
I'm all for moving forward and I stand by the devs and the Joomla team on that one, however I disagree with simply dropping 1.5 extensions from the database.
To be slightly critical, it's hard enough finding certain resources sometimes with Joomla, the last thing you need is a seperate site \ location for 1.5 extensions.
There is also a huge number of people using IE 6 but Microsoft clearly dropped support for it.
That's actually incorrect as per Wikipedia.Microsoft has themselves, despite admitting to some of its many flaws, stated that they will support IE6 until Windows XP SP3 support is removed, meaning IE6 will be officially supported until 2014, 13 years after its release. However they have now started their own campaign to encourage users to stop using IE6.
Note the word encourage and not forcefully remove.
EDIT:- Just another couple of examples and note this isn't flaming but maybe just thoughts others haven't considered.
Looking at the FLEXIcontent product it has 175096 views, it's currently only marked for Joomla 1.5. I believe, from what I have read, they are still working on a 2.5 version. At what point of stability a product needs to be to add the magic 2.5 icon I'm not really sure. As it stands though this product will loose a presence in the directory. So I guess it's down to the developers of that product to update their listing, but what if somebody is still working on a 2.5 version that isn't ready to be listed yet? They will lose all their existing reviews and credibility.
Second example, how about non supported 1.5 products that developers are willing to take on as new projects, (extensions and forks). Many a time have I seen a developer state that he wanted an extension but couldn't find one for 2.5 so he has re-written a previous 1.5 one. Without seeing what was previous available may kill off such a thing.
Just a couple of things to consider.