Update 6.10 release notes

Author: rbuch

CHANGES

@@ -32,15 +32,18 @@ Misc:
   infrastructure. Programs that rely on custom load balancers or the internals of the LB
   infrastructure will likely require some changes for compatibility.
 
-Known Issues:
+- Upcoming deprecation notice: The next release of Charm++ will remove the BigSim emulation
+  facility from the runtime system.
 
-- A hang occurs in multiprocess runs in SMP mode with completion detection on GNI builds.
-  (https://github.com/UIUC-PPL/charm/issues/2508)
+Known Issues:
 
 - Recent InfiniBand machines crash in SMP builds due to problems in the verbs layer implementation.
   Users are recommended to use UCX for the time being if possible.
   (https://github.com/UIUC-PPL/charm/issues/2532)
 
+- UCX sometimes hangs/crashes on Frontera.
+  (https://github.com/UIUC-PPL/charm/issues/2635, https://github.com/UIUC-PPL/charm/issues/2636)
+
 Charm++ Features & Fixes:
 
 - Support for a new Unified Communication X (UCX) networking backend in LRTS,