1.21.9 added a the DebugRenderer.initRenderers method that gets called whenever a debug renderer might be activated or deactivated. This method then checks the conditions for each debug renderer and collects all active debug renderers in 2 lists: debugRenderers and lateDebugRenderers. Rendering then happens by simply calling all debug renderers in the list. This is a performance improvement over the behavior in 1.21.8 where the conditions for each debug renderer were checked every frame.

In 1.21.8 debug renderers weren't any different from anything else being rendered, so no special api was necessary, as it would just duplicate WorldRenderEvents.LAST. However, now it's reasonable to assume that mods want to also make use of the performance improvement and add their debug renderers to debugRenderers or lateDebugRenderers.

Note: 25w41a removed the lateDebugRenderers field, but the rest remains the same.