The fact that you have to ask if there is any illegal data on chain says that even if there is it’s not obvious and not natively supported by the network. Any illegal data mined in the op_return part of the transaction will be known about because:

1. It is a full contiguous string of data that can potentially be dragged and dropped into video playing software without any complex reformation of data

2. The reason op_return was created in the first place was for a way to include a very small amount of arbitrary data in a transaction, meaning arbitrary data is op_returns purpose. Therefore increasing the default op_return limit 1250x signals that bitcoin now supports and facilitates the relaying and storage of arbitrary data which must include illegal data as well.

As soon as illegal data was mined in bsv using op_return it was known about…