And before Pidgin was named Pidgin, it was named GAIM, which was short for GTK AIM, which was short for GIMP toolkit AOL IM, which was short for GNU Image Manipulation Program toolkit America Online Instant Messenger, which was short for GNU's Not Unix Image Manipulation Program toolkit America Online Instant Messenger and it never ends.
I could be wrong but my understanding of the EU interoperability laws is that they cover interoperability between services (for example, being able to message a Telegram user from WhatsApp and vice versa), not interoperability with third-party clients apps.