I Know that event receiver should be deployed as a sandbox solution. But what if I deploy it as a farm solution. Is there any negative impact or not.
There is no requirement to deploy event receiver as sandbox solution, it is neither a best practice. SP 2019 doesn't support any longer sandbox solution so if you you want it to be future proof you shouldn't use sandbox.
The advantage of sandbox solution is that the execution happens in different process than the the rest of web app. So if you have issue in your code like a memory leak your web app process will not be affected only sandbox process will show increased consumption of memory.