@CerstinMahlow You're basically almost right on time with that question! 😉 When the Digital Markets Act goes into effect soon (May I think?), it will be mandatory for large messengers to be interoperable. https://www.theverge.com/2024/2/6/24063705/whatsapp-interoperability-plans-eu-dma
I run a Matrix server that I'm planning to use to talk to people on Signal and WhatsApp. Last I heard, Apple was still trying to weasel out of it for iMessage.