Wither still can be stucked under End exit Portal or can be stucked on End cities portals. Not directly, at end cities portals, but combining determined blocks, minecarts and stuff around it, It will be stucked.

When this happens, at least with wither, the game should detect this issue and instant move the wither some blocks away from bedrock to release them and force the player to fight it.

I know that change and fix this will break so many automated farms with wither, but I think that this behavior is a bug and I think that wither is intended to help you to farm wither roses killing enderman, farm obsidian or farm wood.

This bug is at the same level than duping objects bugs.