Is there any alternative way to become polite to say this sentence?
Solution 1:
Don't forget to maintain social distancing.
"Don't forget" implies that the audience already knows they're supposed to maintain social distancing, and is generally perceived as a polite reminder rather than an order.
Solution 2:
'Remember to maintain social distancing' 'Please remember to socially distance' or 'Please ensure social distancing'.
I have kept these deliberately short because of the feedback that 'Don't forget to maintain social distancing' might be too long.