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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
F-Droid has a number of apps.
KeePassDX (also on F-Droid) also supports TOTP, as does KeePass 2.0 on desktop, if you're comfortable keeping it with your password manager.
PyOTP contains plenty of information about how to implement an authenticator app.
https://pyauth.github.io/pyotp/
It’s incorrect - Apple has in fact implemented attestation as can be seen in https://github.com/lbuchs/WebAuthn.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.