12 years of experience with .Net here - it’s been a long time since I used log4net and I was never intimately familiar with it, but I’m not aware of any built-in or common .Net functionality that will make a web request and remotely load code just by parsing a string. So unless the log4net library totally implemented that feature from scratch, it should be safe.
What about 'Assembly.Load(bytes)' or 'Assembly.LoadFrom'?
No reason to think that it is -- it's related to a specific implementation of a java technology, it would require completely a completely hypothetical parallel track.