https://cryptpad.org/
https://cryptpad.fr
Provides functions such as Excel spreadsheets, rich text documents, markdown slides, whiteboards, forms, code editing, task boards, etc. High encryption algorithm, anonymous. Provides traditional Chinese and more than a dozen other languages. Supports project encryption. Free registration, content permanently saved; resources of visitors are also supported and saved for 90 days without access.
Can be used as a team collaboration tool, a tool for managing learning notes on multiple platforms, and so on.