Running this command below in 1.15.2 would tp any player who is in the Overworld(Dimension:0) to the Nether at 0 64 0

/execute in minecraft:the_nether as @a if entity @s[nbt={Dimension:0}] run tp 0 64 0

Running this command above in 1.16 Release Candidate 1 will not produce any syntax errors and not tp any players to the nether at position 0 64 0

Additional:
Running this command below with the Dimension:0 removed will work as expected. i.e it will tp all players to the nether at position 0 64 0

/execute in minecraft:the_nether as @a if entity @s[nbt={}] run tp 0 64 0