Some popular plugins stopped working in 9.0

This issue has already been raised in the Plugin Users forum, but no one from JetBrains has responded.  I do not recall any official announcement from JetBrains mentioning any major changes in the plugin API and the 9.0 requiring plug-in updates - compared to version 8. However, some plugins that used to work fine in Idea 8 fail to install properly in Idea 9. (For example, the Regex plugin, and a number of other useful ones.) Why?  My question is: does 9.0 require the plug-in authors to re-work their plugins, or is it a bug in IDEA?