Libxml has been v2 seemingly forever, no doubt some v1 installations are out there, but the libxml tag is used synonymously for libxml2 on StackOverflow.
Update this is still outstanding - anyone else like to support this proposal? At the moment libxml has 123 instances and libxml2 296.
Update 2 a year has passed - still no synonym.
Update 3 another 6 months more have passed - now 196 v 477 instances ...