Following up on this, since I found the thread looking for something else:
Support for characters outside the BMP range (i.e. support for UTF-16 surrogate pairs and 4-byte UTF-8 sequences) is being added for config files (including toolbars) and script add-ins.
The config file change is already in 12.9.2 beta, while the script add-in change is coming in 12.9.3 beta.
Scripts that use UTF-8 (whether BMP or not) should still have a BOM (byte order mark) at the start to indicate they are UTF-8. That hasn't changed. We've just added support for decoding 4-byte sequences. (Which were very rare, at least in western languages, but are increasingly common with things like emoji.)
Prior to the change you would have see "Trunk ? Branch" in the name column for the selected script.
Also, tangentially related, here's a command for adding/removing/toggling BOMs on files, in case it's useful: EditBOM - Command to add, remove or toggle UTF-8 byte-order-mark