Indeed that specific UI isn't there anymore about individual game updates, but you can still view updates by games individually via the new UI. A "better" method than the one I suggested would be to:
1) Click on "View", then select "Small Mode".
2) You'll then see the old style games list UI, and from that UI you select the game you want to see the updates news about by right-clicking on it/them.
3) When you right-click on the said game(s) you'll have a new small window with new options, from there chose "View News" (and why selecting the news from the UI of the Small Mode is because it acts like a filter that would otherwise wouldn't be possible to apply as far as I know, a filter that compiles all the related news to the game).
4) Let's say you right-click on Portal, then select View News, it will then bring you back to the regular newer big UI window of the News section for Portal... BUT... from there you'll have a "Full Stories" and a "Headlines" filter to the left side of the column that is to the right side of the window where you can see options such as "Channels", etc., so select "Headlines" instead because by default you'll have Full Stories.
5) By searching your game-specific news via the Headlines method it just simplifies everything (at least as far as the new UI is concerned). Then in the Headlines you'll have to look for the «Game Name» Update Released ones, and those actually do contain the "change log"-style update news that you're looking for.
So, in other words, it's still possible to see what changes in what updates for specific games, but you have to go through a longer research process thanks to a less effective GUI (certainly better looking, but not quite as fast and just effective as to "get the job done" as it was before).