Hello I just tested it and this bug no longer works in v1.16.40

This bug works on Creative mode and Survival mode.

You can swim on trapdoors.

How to do it (Screenshot):

Place a trapdoor, under it place another trapdoor and then put water in them.

Put some trapdoors infront of the water and try to swim.

You will be able to swim on them even though there's no water in them.

If you jump or you touch another block you will go back to normal.

I couldn't upload the video here because it's more than 10MB so I uploaded it on Youtube: https://youtu.be/kw5fqo6hzqU