Despite recent events, I don’t see this happening. Signal has a strong reputation as a privacy-supporting app without any weird fringe Internet subculture attached to it. And I’m unaware of any public groups like you find on Facebook or WhatsApp.
De-platforming a private messenger like Signal would be a strong signal against an open, unmonitored Internet and raise too many eyebrows. Amazon would be seen as complicit with government for not allowing citizens to communicate freely and privately with each-other.
If Signal adds features to let users post extremist views publicly, that’s a whole different ballgame and the cards are off the table.