Rules guidance says the DM cannot arbitrarily destroy your magic items in Adventurer's League.
Magic items cannot be destroyed at the whim of the DM. Just like its not up to the DM if a character's body is destroyed, both cases must be called out by the creature or adventure to be possible. This prevents a vindictive DM from permanently destroying an item or character and puts the fate of the character and their hard won magic items in the hands of the owning player.
According to the D&D AL FAQ v6.1, any official rules guidance posted to Facebook is official:
Any rules guidance given by an Admin using the #AL_Admin or #AL_Official hashtag is considered official rules guidance. In time, it may be added to this FAQ and made available without the drudgery of having to resort to Facebook’s “search” function.
Mending a magic item?
If a magic item is broken, mending specifically cannot repair it, according to the spell description (Player's Handbook p.259):
This spell can physically repair a magic item or construct, but the spell can't restore magic to such an object.
This implies that a broken magic item loses its magic, although strictly speaking there's no rule that this is the case. It's entirely up to the DM whether a broken magic item can be repaired.