Strictly speaking:
- xotcl (the language/tcl extension) doe not depend in any way on libthread.
- xotcl-core (the openacs package) supports libthread optionally
- if you want to you use e.g. background-deliviery or the xotcl-request-monitor, you need it.
see e.g. the entries tagged with libthread in the wiki: https://openacs.org/xowiki/tag/libthread
