1.16.5 is a very small release that fixes only two issues. However, these issues are so major that Mojang has hidden the information about them from public view.
The patch notes for this update only mention that
two critical stability issues were fixed, not going into any further details.
These issues are both related to exploits that allow players to crash a server using very simple mechanisms. I will not detail these exploits here, but, for example, one of them relates to using an item on a single block to crash a server.
The exact details about these exploits are hidden from public view on the official bug tracker because it may take some time for all servers to update to 1.16.5, and Mojang does not want to implicitly encourage the use of these exploits on servers still running 1.16.4.
Servers do not need to update immediately, however, as 1.16.5 clients are compatable with 1.16.4 servers, meaning you can connect instantly to servers that haven't yet updated even if you are playing on the new version.
If you were expecting a new snapshot for the upcoming 1.17 Caves & Cliffs update this week, Mojang aims to release one next week, so stay tuned for some cool new features to be added soon.