I think Pidgin should fit the bill. Awesome client with a lot of useful plugins.

Gajim [1] implements a lot of modern XEPs (directly or through easy to install plugins), including OMEMO. I don't know how well it works on non Linux systems (they provide Windows binaries at least).

[1] https://gajim.org/

Yes, feature-wise Gajim is one of the best. Unfortunately the UX is pretty bad :(

Indeed Gajim is quite stuck in the past, and I regret to say this as it hurts me, since I used to be a hardcore fan.

Conversations indeed is very decent.

it works. doesn't feel very native, but then neither do a lot of the big names in chat right now

It is worse, actually, because it uses GTK, and is only available on Linux and Windows (not macOS, although there have been attempts).

Most of the "big names" who provide modern and contemporary desktop IM apps deliver on the 3 platforms (Linux, macOS, and Windows), and use mostly web "technologies" (sometimes "pure browser", and sometimes with electron-like desktop "packaging")... so as a consequence, it is very native, indeed, but it is a generally consistent and continuous UX (user experience) between mobile/tablet and desktop/laptop.

There is dino: https://github.com/dino/dino

It's still very early in the development but it looks pretty good.

I don't think it's cross-platform, though. At least, not yet. (Would be happy to be mistaken)

Is GNU compatible and thus can run on all platforms (e.g. Windows via MinGW). Some features might require special handling for Windows / OS X (GPG, Audio/Video) and Linux is for now the "official" target platform...

https://swift.im/ is cross platform client that strives to do things right. Some of the more modern features are available in the latest beta.

Swift implements - like Gajim discussed above - a very old and outdated UX, and thus cannot be relevant these days.

I speak harsh words, and I am sorry for this, but I was expecting much from Swift at its birth, but that was not delivered over time.

Speaking of modern XMPP... Is there a way to have a shared history for a group these days? Say the server has three users — A, B and C. If A and B is chatting and C is not currently present in the chat, can C see the conversation that took place in the group chat when C was not present when she returns?

Yes and no. The only production-ready of such a requirement is "MUC light". Basically, it is a very simplified version of MUC, that does not rely on user's presence. It exists in the MongooseIM server, in Smack library for Android and in XMPPFramework library for iOS, as well as in Mangosta iOS and Mangosta Android, two free/libre/opensource clients soon to be released. So it is ok to develop a solution based on this approach.

The "open extensions" is available here: https://mongooseim.readthedocs.io/en/latest/open-extensions/... We call this "open extension" because it was not accepted by the XSF, but it is very close to an open standard.