The short answers is - we don't plan to support IE11 in Telerik UI for Blazor. We are aiming at modern and supported browsers like Chrome, Firefox, Edge, Safari.
Client-side Blazor apps (that is, those relying on WebAssembly), are not not supported under IE, because it does not support WebAssembly.
Server-side Blazor apps (that is, constant WebSockets connections to the server), while not requiring WebAssembly, are likely to cause issues too. The problem then arises mostly from the JS Interop code that is needed. IE 11 is not a modern browser and does not implement the current JS standards and so errors will be thrown. We do not intend to add polifylls for it.
Progress Telerik UI for Blazor