Shadowban
Shadowban is the popular term for an account's reach dropping sharply without any notice, and Instagram does not use or acknowledge the word.
What it does not tell you
Two real things sit underneath it. Instagram does limit the distribution of content it judges to break its recommendation guidelines, and it does tell you: the account status screen in the app shows whether anything you posted is not eligible to be recommended to non-followers. The rest of the time, a sudden reach drop is far more often a change in what you posted or a normal swing than a penalty. Check the account status screen first, because it turns a rumour into a fact you can act on.
Now the harder question. What do yours say?
Ask Synclify in the same plain words and it reads your own account. Every dashboard and chart in it is free to use on every plan.
Cancel any time. Every dashboard and report is free to open.