It's in a doc for eZ 3.9: http://pubsvn.ez.no/nextgen/trunk/doc/features/3.9/ezsubtreeremove_script.txt
The changelog of 3.8.4 (<i>http://ez.no/download/ez_publish/changelogs/ez_publish_3_8/changelog_3_8_3_to_3_8_4</i>) mentions some related INI setting: <i>Added content.ini[RemoveSettings].MaxNodesRemoveSubtree Maximum number of nodes to remove (using Remove Subtrees feature)</i>, so I guess it was added in this 3.8 release.
i think , in this case deleting should be done directly in database, because deleting, when you need to create ph objects, to make thousands of queries takes a lot of time. Any extension which i try, and which is using ez php framework - gives me timeout...
For example, i need to delete 22 000 nodes. No extenstion at the moment can do this. With PHP - forget about it...