This basically brings your account up to date with WoW's story so far/current state in preparation for the upcoming sixth expansion, Legion. It might also be a move to stop people thinking "well, I'm an expansion or so behind so I'm less likely to pick up the new one". Anyway, if you're not sure what a Warlord of Draenor is or whether we liked it, we have a Wot I Think about the expansion from when it launched.
This move was confirmed by a customer support rep via Reddit, although there's a bit of kerfuffle about the removal of Warlords of Draenor from sale because it means you can no longer redeem registration codes for copies of the expansion bought but not activated through Battle.net yet. This isn't a problem in terms of access to the content of the expansion, but it does mean some players are worried about what that means for the level 90 character boost which was included as part of said expansion.
A Blizzard customer support employee advises players in that situation to get in touch via the game key problem option of customer support. If you activated the key but haven't used the boost it should still be in your account.
It's interesting that there seems to have been so little messaging around the actual switch from a paid expansion to being part of the base game, though. I'm reading around it and the information seems to have come through in dribs and drabs. For example there's a forum poster saying staff in their local retail store said they'd been asked to send copies of the Warlords of Draenor expansion back and the expansion would be free, but when they followed up with a game moderator they didn't know about it. There's not an enormous gnashing of teeth or anything, it just seems like the switchover has been lacking a little in clarity.