In Minecraft versions 1.9 to 1.19, trapdoors could be climbed when placed in line with ladders. In version 1.20, this behavior has been removed or broken. There are existing bug reports concerning trapdoors being climbable, and it's possible this change was made in response to those reports.

Motivation:
When climbing a ladder up to a floor with a trapdoor and opening it, if there's another ladder above the trapdoor, it can be challenging to climb through or jump to the next floor, as the player might be blocked by that ladder. As a result, the ladder often needs to be removed to access the floor and then replaced to climb further. This workaround can be cumbersome. To avoid this issue, players might resort to replacing the trapdoor with a ladder that remains open. This behavior suggests that players value the trapdoor's former functionality and would benefit from its return.

Suggested Solution:
To improve user experience, a visual indication could be introduced to signal whether a trapdoor allows climbing based on its position relative to a ladder. When a trapdoor is closed, players should see the ladder through it if it's aligned with one, indicating that climbing is possible when opened. If the trapdoor is placed at the top of a ladder, it should visually connect with the ladder below; if placed at the bottom, it should connect with the ladder above.

To avoid confusion when trapdoors are placed alongside other climbable items like vines, the trapdoor should always allow climbing if it's connected to such items.