I suppose we're talking about deprecated assets that you've downloaded from the asset store. The Package Manager doesn't have an option to hide deprecated packages. The only way to do this (at the moment) is to directly hide it from the asset store.
On this post (Remove assets from "My Assets"?), user APSchmidt said:
There is a "hide" button, just under the "Open in Unity" button. You must go on the assets store here, not within Unity to find it.
Go to AssetStore/Account/Assets and find the asset you want to hide. You'll see three options:
- Add Label
- Hide Asset
- Open in Unity
Click on "Hide Asset". This should remove the asset from your asset list, thus hiding it on the Package Manager.