"Slow down, please."
Having lived and studied in Taiwan (where traditional Chinese is the primary language) for 9 years, omitting it would sound perfectly normal to a Taiwanese resident. Saying ㄦ at the end of a sentence sounds more like a mainland China accent, but it would still be grammatically correct either way.
（請慢一點）-Traditional Chinese. 11.12.2019
as Swansae said, slow is not a verb in chinese, and so the sentence doesn't work gramatically when it's just "please slow". 一点 means "a bit" or "a little" so the sentence roughly translates as "slower, please". 二 is a personal preference and certain dialects use it more than others. Yes, 下 translates to down but can be used differently in different contexts.