The Signal messenger and protocol.

I mean the creator requires the Android/iOS primary device or no service… by design. They do not want another option.

Also not every project is open to contributions that fundamentally change things—which is why forks are so common in free software. Changing this core isn’t worth the effort since there are alternatives that already do everything Signal does, but just have a worse marketing team. I am very much allowed to be as angry at family stuck on Signal as I would if they used WhatsApp for similar reasons—both technical & social.