Yes, but many legacy .Net apps were delivered without source-code, the original developer is not available, and now people like OP and me are looking to run this old crap (for paying customers) without having to maintain a dedicated Windows VM.
Have you tried decompiling it? Telerik Just Decompile will decompile it back out to C#. It ultimately depends on agreement you had with the original developer as to whether it is legal or not.
Even with source, it's still a big undertaking to convert to .NET Core but efdee is right, it's just postponing the inevitable. I'm sure we will go to .NET 5 soon enough.