Comment 2 for bug 53271

"supported" not "cannot open it"

I can think of two possible meanings here.
One is the nested tree/rich root support stuff. e.g. "bzr join foo" will fail because the operation is not supported on plain trees.
The other is for old formats we still upgrade from but won't do other things on - e.g. a bzr 0.0.4 format tree. Both are situations we can 'bzr upgrade' from.