I agree that EteSync is still the best solution for syncing contacts/calendars/tasks privately, regardless of how recently the apps have or haven’t been updated. Proton Mail doesn’t actually encrypt your contact names and email addresses.
When compared to the other recommendations on the website, it’s the only one that is actually syncing your information to your device, rather than just providing an app to access it (Proton Mail’s contacts for example, cannot be used to store and sync your Signal contacts, which are taken from your phone’s system contacts storage).
It’s also the only open source one. Or at least the server part. Proton Calendar’s apps are currently not open source though, and EteSync is.
Because the server is open source, it’s also self-hostable. This is not to say that you should self-host in order to not pay the very reasonable subscription fee of $24 a year, but for some people self-hosting is important, and if you already have the infrastructure, it makes sense to self-host it.
While it’s not recommended for contacts/calendar/tasks sync, EteSync is also better than Nextcloud’s syncing support. Both sync to the system storage, but this information is synced to Nextcloud in a completely unencrypted form, and it is also stored in an unencrypted form. This is true regardless of whether you use the end-to-end encryption add-on (don’t: Don't Recommend Nextcloud E2EE - #27 by ph00lt0), as Nextcloud simply runs a plain CalDAV and a plain CardDAV server.
While it may be true that “updates” are important for security, and by extension also privacy, there is nothing wrong with the current version of the apps (at least for my use case, which is an Android device, and multiple Linux devices), nor is anything to be gained to use by adding say, Material You support to the Android app. It just syncs your information after all.
To my knowledge, there are also no vulnerabilities in the apps that aren’t being fixed. F-Droid for example, labels apps with known vulnerabilities, and EteSync doesn’t have this label. If the server is being actively maintained, it should also be assumed that the chances of a vulnerability affecting EteSync users is low.
I understand if the team doesn’t particularly care about my opinion, as they have no reason to, but I don’t see the point of considering apps that haven’t been “recently updated” as worse than apps that actually have less privacy, less features, and less usability. I’m in favour of adding EteSync back to the list for these reasons.
Tl;dr: EteSync is still private, open source, and self-hostable. It syncs to the system, where the other recommendations don’t. And as Proton Mail’s contacts aren’t fully encrypted, and their calendar app isn’t yet open source, EteSync is the most private and open source option. Updates aren’t everything.