The risk begins after BIP 91 activates.
Is it still safe to transfer bitcoins from online wallet to offline wallet?
Looks like it! BIP-91 activates in 53 blocks so you still have a few hours.
After that what will happen?
All nodes will stop accepting non-segwit blocks.
Actually, nodes that are enforcing BIP 91 (there are not many) will stop accepting blocks that are not signaling for SegWit.