In 1.21.1, enabling entity hitboxes through F3+B then opening your inventory would not display the player's hitbox in the inventory render when the graphics option was set to Fabulous, however in 1.21.2-rc2, it does.
Here is a screenshot with hitboxes enabled in Fabulous graphics in 1.21.1:

Here is a screenshot with hitboxes enabled in Fabulous graphics in 1.21.2-rc2: