Prior to 1.11, we were able to add entity data tags to entities such that one could say "/summon ArmorStand

" and it would from there on have a tag "Tag:poisonous".

As of currently (1.11.2) and possibly in 1.11 as well, trying to add tags in this way fails as does trying to use the /entitydata command. At first /entitydata seems to work as it says it is successful and the output includes the new tag, however the actual entity's data does not change.

Whether or not this was removed intentionally, command block users used this to mark special properties on entities they were using. While it is okay to use the Name tag, there is only one of those and won't work if they want to apply multiple special properties to them. At the very least, the /entitydata command should fail when an incorrect tag is specified.