As someone who's embraced this world of longer tweets, having my (already) favourite Twitter client (on any platform) now letting me vent my thoughts to the world in extended form is well worth this News post.
The new version is v4.2.501 and here's the screenshot proof:
The fix would have been here sooner, apparently, but the developer, Brandon Paddock, had a source code issue for a month or two and had to re-do the last few bug fixes from scratch before the support for extended tweets could be implemented.
As usual, Tweetium is a commercial UWP application and is about £2.50 in the UK Store, but well worth it, and works on phone, tablet, laptop, and so on. In fact, here's the new version in action on my Surface Pro, again creating a 280 character tweet:
PS. Bonus link: here's the last time we covered a Tweetium update (Sept 2016). Interestingly, I've never formally reviewed it!