In theory you missed that memo in 2019, it's that Corona threw in a curveball that gave us an additional 3 years on the inevitable recession. This is why timing the market is hard.

Alternatively, people I know who got that memo in 2019 sold their stocks, went short and now are in a significantly worse position.

Was there a memo at the end of 2018? If not, why would there have been a memo in 2019 but not 2018?

Yes, the memo came in November 2021 from the federal reserve and multiple companies.

Does a memo get issued for when to buy stocks?

Just follow what congress members do: https://www.businessinsider.com/congress-stock-act-violation...

Yes, wait for your first S&P 500 bankruptcies and that will be your sign that the Fed pivot is imminent.