What are the Consequences of using .NET Framework 4.5 after it's support ended by Microsoft

Thanks in advance for your time. My question is, we have a .net application uses .net framework 4.5. We are installing on this on a windows server 2016, where the .net framework 4.6 is pre-installed. But as we all know .net 4.6 support also coming to an end. But we don't want to upgrade the framework yet to a newer version, But client want some peace of mind that it is still safe to use it. I read the MS documentation and it says, that it's recommended to upgrade but not forced to upgrade. So my question how bad it can be if we don't upgrade, is there any security concerns at the moment or in future. We tested the application on Server 2016 and it works fine. Any feedback is highly appreciated. Thanks.