I'm a big fan of static type analysis and code surfing, and that extends to book margin notes and indexes somehow.
I've heard you loud on clear on the feedback on Literal. I'll have a release out to make sign-in optional in the coming weeks and will follow up here once it's released. I'd be happy to create you some guest credentials in the meantime as well, shoot me an email if you're interested.
I wasn't aware of F-Droid but just created an issue to track publishing releases there here [1]. I'm not sure the inclusion timeline on their side but the work on my side should fast follow guest auth support.
Edit: to clarify _why_ authentication is currently required - the username is used as a primary key on associated data (e.g. annotations), and since I'm actively working on this, I try to personally reach out to everyone via email in order to solicit feedback. You can view the privacy here [2]. I'll move to support an optional auth flow regardless.
I'm currently working on a textual annotation product, Literal [1]. It's open source and implements the W3C Web Annotation spec [2] natively (Yes, there's a web spec for this kind of thing!). You can see a 1m video of how it works here [3]. It's currently Android-only but if you found this article interesting and / or find yourself wishing you could annotate or save bits of text that you read, I'd love to connect with you!
[0] https://www.nytimes.com/column/close-read
[1] https://literal.io/
[2] https://www.w3.org/TR/annotation-model/
[3] https://www.youtube.com/watch?v=nH1ukQY3Ia8