Certainly many people remember the big cleanup on extensions.joomla.org — the day when all extensions for Joomla 1.0.x disappeared from the Joomla Extensions Directory. An official announcement from Joomla indicates that we can expect a similar event to take place this year. Very soon, in fact.
According to the statement, on 30 April, 2012 all extensions (these include components, modules and plugins) for Joomla 1.6 and J1.7 not yet marked as compatible with Joomla 2.5 will be completely removed from the JED catalog.
About six months later, extensions for Joomla 1.5 are to be removed from the catalog. We hope that by then most Web developers will have adapt their projects to the latest version of the the Joomla CMS.
Extensions designed for the Joomla 2.5.x series will remain listed in the directory for 6 months following the Joomla 2.5 End of Life (EOL) period. Joomla 3.x Series extensions will be listed in the directory for 6 months following the EOL of the LTS version in the series (version 3.5).
As of April 1, 2012, the JED will only accept listings compatible with the latest Joomla Short Term Support version (2.5). You may still mark the extension as J1.5 compatible, but 1.5, 1.6, 1.7-specific listings will no longer be accepted.
The number of extensions for Joomla! on 11-04-2012 was 9347 and today their amount was slightly higher 9366. So, within two weeks that number may drop by several hundred.
More info you will find at community.joomla.org