From what I understand, since Microsoft will not be releasing any new versions of .Net Framework, they have committed to supporting 4.8 indefinitely:
.NET Framework 4.8 is the last version of .NET Framework, and no further versions will be released. However, .NET Framework will continue to be serviced with monthly security and reliability bug fixes. Additionally, it will continue to be included with Windows, with no plans to remove it. You don’t need to migrate your .NET Framework apps, but for new development, use .NET 5.0 or later.