Say what you want about Telegram, but it has one of the best data export/backup features I've ever seen.
Fully client-driven, with real-time progress display (and even the ability to manually skip large file downloads on-the-fly).
The output is a bunch of plain HTML pages with just 200 lines of pure JS (no frameworks) for some minor interactivity features, that loads instantly and looks roughly like the Telegram client itself, easy to browse and search.
The JSON is one big blob with all the same data in a trivial format. The text encoding is interesting: Telegram supports rich text, but instead of in-line HTML-style markup, in the JSON it's encoded as JSON objects representing the different spans of text with different format. Very clean.
I'm all for Signal and E2EE and distributed systems and all that, but... Telegram is, by far, the least-bullshit most-fun messenger I've ever used. Everything just seems to work, it's lean, has native open source client apps, a big pile of features that are cohesively integrated and work, API/bot support, useful stuff like automatic translation (premium feature, but that's understandable since translation APIs aren't free), etc.
Other platforms would do well to learn from it.