In 1.21.1 and earlier, setting pitch of display entities to 180 (or any value above 90) allowed creation of "upside down" displays, that could share animation / transformation with other display entities without requiring to compute separate transforms. In current 1.21.2 pre-release it's no longer possible, as it is limited to values between -90 and 90. Rotation of existing display entities is also affected, which can be seen as breaking change.

Example command:

/summon minecraft:item_display ~ ~ ~ {Rotation:[0f,180f],item:{id:"tnt"}}