Transport Layer Security (TLS) is the protocol that powers most of the internet these days. It gives HTTPS the S for “Secure”. F-Droid uses it to keep the connection to repos private. After many years of slow updates and an increasing number of vulnerabilities, there is finally critical mass to stop using the old, broken versions. TLS version 1.2 is not vulnerable and is supported basically everywhere. TLSv1.2 was finalized in 2008, so this is very far from the bleeding edge. TLSv1.0 and TLSv1.1 are due to be officially deprecated by the IETF, the standards body that actually creates the TLS standard. The major browser vendors have all promised to drop them in 2020.
One way to enforce TLSv1.2 support would be to configure the f-droid.org webserver to stop supporting TLSv1.0 and TLSv1.1, like we have with SSLv2 and SSLv3. We prefer to do this on the client side instead, so that old versions of F-Droid and old devices without TLSv1.2 (added in Android 4.1) will continue to work with f-droid.org. Also, doing it client-side means that connections to all repos will gain this protection without changing the repos server setups.
With the NetCipher approach, the latest versions of F-Droid will never use TLS older than v1.2 since they will refuse to connect unless TLSv1.2 is available. Since f-droid.org can still safely support TLSv1.0 and TLSv1.1, older clients and Android devices will still be able to connect, even if they do not support TLSv1.2. It is win-win for everyone.
This will mean that any repo will need to support TLSv1.2 to work with F-Droid client from here on out. If a webserver does not support TLSv1.2, it is really too old to be used safely anyway. Even the oldest supported Red Hat Enterprise Linux release (6) supports TLSv1.2, and that was released in 2010.