A notice is public, filed on an 8-K, and often adds a "D" or "C" indicator to the ticker. It is a clock, not a verdict. Companies typically get 180 days for a price deficiency and may receive a second 180-day period if they meet the other initial listing tests and state an intent to cure, commonly by a reverse-split.
For traders the notice is a dated catalyst. The reverse split usually arrives in the final weeks of the cure period, and the stock frequently sells off into it.
Example: notice received 1 February with a 180-day cure ending 31 July. No compliance by late June, so a 1-for-20 reverse-split is announced for 10 July, taking a $0.22 stock to $4.40 and curing the test on 24 July.
Related: listing-standards, delisting, reverse-split, otc-markets, uplisting