Ah, looking more closely at the XDG base directory spec and other pages about it, I think byobu should be looking for user configuration files under ~/.config (not ~/.local/share).
From the spec, "$XDG_CONFIG_HOME defines the base directory relative to which user specific configuration files should be stored. If $XDG_CONFIG_HOME is either not set or empty, a default equal to $HOME/.config should be used."
If you consider the stuff in the old ~/.byobu to be closer to data files than configuration files, then looking under ~/.local/share makes sense, but you should check user-specified $XDG_DATA_HOME instead of $XDG_CONFIG_HOME.
Ah, looking more closely at the XDG base directory spec and other pages about it, I think byobu should be looking for user configuration files under ~/.config (not ~/.local/share).
From the spec, "$XDG_CONFIG_HOME defines the base directory relative to which user specific configuration files should be stored. If $XDG_CONFIG_HOME is either not set or empty, a default equal to $HOME/.config should be used."
If you consider the stuff in the old ~/.byobu to be closer to data files than configuration files, then looking under ~/.local/share makes sense, but you should check user-specified $XDG_DATA_HOME instead of $XDG_CONFIG_HOME.