Chat apps are browsers, and Discord keeps every version it downloaded

Both are web applications in a native wrapper, so both store what a browser stores. The difference is what happens when they update.

Where they are

Get-ChildItem "$env:APPDATA\Slack","$env:APPDATA\discord" -Directory -EA SilentlyContinue |
  ForEach-Object {
    $s = (Get-ChildItem $_.FullName -Recurse -Force -File -EA SilentlyContinue | Measure-Object Length -Sum).Sum
    [PSCustomObject]@{ MB = [math]::Round($s/1MB); Path = $_.FullName }
  } | Sort-Object MB -Descending | Select-Object -First 12

Slack: one cache per workspace

Every workspace you have joined gets its own cache and service worker storage, and leaving a workspace does not remove it. A machine signed into five workspaces over three years is holding five sets.

Slack has a built in fix, which is the safe route: from the menu, Help, Troubleshooting, Clear Cache and Restart. It signs nothing out.

Discord: every version it ever downloaded

Discord updates by downloading a complete new build rather than patching, and the old ones are not always removed. Look for version numbered folders beside the caches: each is the size of the whole application, and on a machine that has had it for a couple of years this is usually larger than the caches.

Get-ChildItem "$env:LOCALAPPDATA\Discord" -Directory -Filter app-* -EA SilentlyContinue |
  Sort-Object Name | Select-Object Name, LastWriteTime

Everything except the newest is dead weight. Quit Discord from the notification area, remove the older folders, then launch it once to confirm it starts.

Files you downloaded from either

Attachments you saved do not live in these folders at all: they go to your Downloads folder, and a year of shared decks and clips adds up faster than any cache. That folder is worth the same pass.

The same shape elsewhere

Teams has the same layout with an extra wrinkle, and the folder all of it sits in is AppData.