After the implementation of reply protection, is it not possible to send bitcoin from BTC to BCH network , and vice versa?
This was never possible.
A lot of people confused their BTC/bcash addresses and sent their coins to an address (on the same chain).
This was a problem for exchange deposits, who were only monitoring the corresponding chain for this address.
Replay protection was implemented to distinguish between BTC- and bcash transactions.
Before replay protection it was possible to take a (feshly broadcasted) bcash transaction and broadcast it into the btc network (and vice versa).
This would have led to 'the same' transaction being mined on the other chain.