Why do some indicator icons have gtk2 and gtk3 versions, while others have only a single version that works with both libraries?

Question

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?

Answer 1

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.