The Arduino Nano Every works great, but the language/autocomplete support for the board in the Arduino IDE is missing. This has already been documented here. The general symptoms are that autocomplete doesn't work, and suggestions show up grayed out and strikethrough/crossed-off. According to the Github issue, this also applies to other boards using the "megaAVR" architecture, such as the Uno WiFi Rev2.
If you're like me and enjoying using the Arduino Nano Every, but can't live without autocomplete, there is a quick fix. Download the latest version of clangd, and replace the current version in your IDE. Assuming you installed in the default directory on Windows, this is located in "C:\Program Files\Arduino IDE\resources\app\lib\backend\resources\clangd.exe".
I don't know if this is advisable, so someone with more knowledge can comment on it here, but I wanted to help anyone else in the same position. I was frustrated while writing code without autocomplete, and I think others would feel the same. Hopefully this helps!