As you can see from the responses on the WoW from jack's blog

According to Wowhead, players have discovered that many dungeons of Burning Crusade Classic have seen changes to WoTLK Gold enemy behaviors that are essentially preventing them from being grouped up or kited around. They will instead transfer to players trying to make use of walls and ledges in order to avoid harm. Others are becoming immune to various effects of crowd control, like stuns and slows after duration of time. Blizzard also introduced an XP penalty for lower-level players grouping with higher-level ones as a way to stop boosts.

The changes take place at the same time that Blizzard introduced"Joyous Journeys," a "Joyous Journeys" event that grants 50% bonus XP to the time of the release of Wrath of the Lich King Classic on September 26.

They are alike to those that were implemented in WoW Classic's Season of Mastery last year, but now also affect Burning Crusade Classic and, according to an Blizzard forum thread from a few weeks ago. It also affects the upcoming Wrath of the Lich King Classic. Blizzard notes in that same blog post that the gold gained by those offering boosting services is often used for "illicit practices," in which the majority of money earned by boosters being sold to players through third-party websites, which are in violation of the game's conditions of service.

As you can see from the responses on the WoW Classic subreddit, players mostly, seem happy with the change. Boosting became the most popular method of leveling up new characters, and resulted in players quickly gaining level faster than intended. Since so many players utilized boosting to reach max-level quickly and quickly, those who leveled their character the traditional way struggled to find other players around the world to group with for quests or dungeons which in turn pushed even more players to turn to boosting.

There are, however, several players that are dissatisfied by the changes. Certain players who took advantage of boosting services and made many gold in making the same transactions are unhappy however, a lot of Mage or Paladin gamers that didn't provide boosts but did go on solo dungeons to try to ensure that gold didn't feel punished by Blizzard's updates. Those players have pointed out that it's not right for buy WoTLK Gold Blizzard to eliminate boosting while they offer free character enhancements through the in-game store.