Slack, Google and similar tools are not encrypted: authorities can request the hosting companies to hand over the documents, user list and the chat logs. Even if you delete a message on Slack, we cannot be sure Slack actually deletes them from their servers or their backups.
It is important to know that regular phone conversations or popular VoIP tools like Skype or Google Hangouts have wiretapping capabilities built-in. Authorities can request Microsoft to record and hand over conversations with a warrant.
Nextcloud is a secure replacement for the google collaboration ecosystem – Share and collaborate on documents, send and receive email, manage your calendar and have video chats without data leaks. As fully on-premises solution, Nextcloud Hub provides the benefits of online collaboration without the compliance and security risks.
Nextcloud is self hosted allowing you to control where data is stored. This is more complex or you can rent a hosted service.
Nextcloud is the system used by Extinction rebellion for their digital collaboration