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.