Default text-to-speech (TTS) engine in LineageOS 16.0?
Pico TTS is unmaintained and dead. At least publicly. Presumably, the fixed version of it is what is now called "Googls TTS", which is no longer open source.

My suggestion, if you're looking for an open source TTS, is to use Flite TTS, which is available via F-Droid repository or from github.

https://f-droid.org/en/packages/edu.cmu.cs.speech.tts.flite/