Amethyst is not actually free / open source software
Upstream report: https://github.com/vitorpamplona/amethyst/issues/378
The latest version of Amethyst (and presumably all other versions released after around 3 months ago) includes a
PRIVACY.md file and points to it in a mandatory terms-of-service-acceptance checkbox on first app start. The first two paragraphs are relatively reasonable (clarifying that Amethyst does not store user data except as strictly necessary and that it has no control over what remote relays might store), but the third introduces non-free license terms:
You cannot use Amethyst to submit Objectionable Content to relays. Objectionable Content includes, but is not limited to: (i) sexually explicit materials; (ii) obscene, defamatory, libelous, slanderous, violent and/or unlawful content or profanity; (iii) content that infringes upon the rights of any third party, including copyright, trademark, privacy, publicity or other personal or proprietary right, or that is deceptive or fraudulent; (iv) content that promotes the use or sale of illegal or regulated substances, tobacco products, ammunition and/or firearms; and (v) illegal content related to gambling.
The fourth also makes the app vulnerable to additional non-free license terms being retroactively added to existing installations of the app without any proactive attempt to notify users (let alone any real opportunity for said users to not consent to the additional terms):