Futu's clients are mostly outside China (in SG &HK)

If you read all of Futu's quarterly reports you will note that a large majority of their clients are outside China. So even in a worst case scenario they won't suffer too much...

Futu doesn't do segment reporting so you can only estimate the percentage of clients. (from quarterly reports) From my estimation, about 70+% of Futu's clients are outside China.