Summary:

Is impossible to build above the nether roof

Steps to Reproduce:

Create a creative world

Go into the nether

Break into the roof

Try to build there

Observed Results:

Maximum height prevents building above the nether roof

Expected Results:

As for Java Parity, it should be possible to build above the nether, as a great place to build and travel. It is a huge part of Minecraft Java Edition that is not in the Minecraft Bedrock Edition.