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In some senses, low-key is marked as figuratively, slang, US, colloquial. Watch for register when choosing this word.
VERBS
be, seem
keep sth
She decided to keep her promotion low-key and didn't tell many people at work.
ADV
extremely, very
They decided to keep the birthday celebration extremely low-key because of the pandemic.
fairly, quite, rather, relatively
The wedding was quite low-key, with just close family and a few friends attending.
deliberately