For example, in Ubuntu we have:
- indicator-application
- indicator-application-gtk2
- indicator-sound
- indicator-sound-gtk2
but other indicators have only a single version that works on both:
- indicator-cpufreq
- indicator-weather
still others have no gtk2 version and will NOT work with gtk2-based desktops:
- indicator-messaging
Why do some indicators need to be maintained in two versions while others do not?
The reason is that indicators like indicator-cpufreq and indicator-weather use an abstraction provided by indicator-application, which also has a gtk2 version. The gtk2 versions are slowly being phased out, but some still remain.