Deprecation is something that is not meant to be used and is not recommended to do so as there could be a much better method of it or it doesn't look alike to the other methods such as capitalization.
Items are deprecated as I said in the other paragraph there could be another better method of it or it really doesn't match the other items in its structure. An example would be like KeyDown which is superseded by UserInputService.InputBegan. KeyDown had a flaws such as it wont listen for keys that are roblox hotkeys like the "/" key which is used for opening the chat. Usually when roblox deprecates something, they provide an alternative.
Deprecated items or methods can be removed by Roblox, which can cause a disaster if all your scripts are using it, in which you'd need to go to every script and remove it and replace it with the alternative. Using items that are not deprecated can make your code much more cleaner and appealing than code that is filled with deprecated methods.
:connect(), instead use :Connect().
:Remove(), instead use :Destroy(); also note that :Remove() just sets
the parent to nil, so you can in fact reparent it back, but :Destroy()
locks the parent. if you'd want to set a objects parent to nil, instead
use Instance.Parent = nil.
The DataPersistence service, instead use DataStoreService.
BasePart.FormFactor, there is no alternative to this.
HopperBin, instead use the Tool object and set RequiriesHandle to false.
:disconnect() and :wait(), so instead just use :Disconnect()