There is no such thing as "slackware-current" yet. Slackware-current is the name of the development tree that will at some point become the next release of Slackware. When the new development cycle starts, the current content of slackware-current/ChangeLog.txt will be erased and new changes will be written to it. Each slackware-current/ChangeLog.txt will ultimately end with "Slackware X.YY is released".
The ChangeLog.txt for Slackware 11.0 is being updated because Pat has already begun adding patches to Slackware 11.0, and this is a process that will continue for a long time to come, probably years.
It never harms to check out the additions to this ChangeLog.txt in addition to the one in slackware-current.