I found this old issue on GitHub:
I’m wondering if there is still a plan to integrate support for segwit deposits on the Powpeg federation address?
Currently if a user has their BTC in a segwit address (or any other non-P2PKH address) then to transfer to RSK they first have to move their BTC to a P2PKH address then make the deposit. And the same in reverse, if when they transfer from RSK back to the mainchain, and they want to ultimately store their BTC in a non-P2PKH address, they must first transfer from RSK to a P2PKH address and then send on to the final non-P2PKH address. This means non-P2PKH users pay twice as much fees and put twice the amount of load on the blockchain as a P2PKH user.
I would like to see support added for deposits and withdrawals using non-P2PKH addresses to lighten the user’s fee burden and the load that they are putting on the bitcoin blockchain.