Players of level 51+ will need to have their horses fully equipped when entering shows.
The exception is on the club shows. Your horses will not need tack, regardless of your level.

Note that the higher the level of the show is, the higher the chance of the tack to break a little.

As a side note, it might not be obvious yet, but this change will likely influence the way you have to play the game. You will need more tack than before, so you should either work on your manufacturer, or find a good one to provide you with tack.
While these changes will pose some challenges on higher leveled players, it also makes it more interesting and less likely to become boring.
Enjoy it.