The update notes you are looking for are not usually present on the tumblr, as you noted. There are two places, however, where you can find the information you seek.
Official Mojang blog
They can, however be found in the official Mojang blog on their main site. The older posts can be hard to find though, but should be able to find them with some googling, I suggest searching for
site:mojang.com -site:bugs.mojang.com minecraft [version]
to limit the results to actual results. Note that these posts are often updated, even changing the title in some cases, such as the one for 1.8.2.
Sadly, the information about bugfixes is often scattered among multiple posts for pre-releases as well as the main post, making them hard to follow. They also seem to be incomplete.
The wiki's version history
A much more convenient way of browsing the changes is offered by the wiki, in the Version History article, neatly sorted by major and minor version. There is an extra page for development versions as well, if you want to check out the snapshots or pre-releases.
The downside is that this information is user-curated and takes some time to be gathered, especially for non-obvious changes, such as the blacklisting of player head textures not hosted on a mojang server in 1.8.4.
Restart your computer then start the launcher again.
I guess the installer should include a warning about unfinished installation and that a reboot is required...
Best Answer
The development versions of the Java Edition are called "snapshots", and have a versioning format like "19w05a", which states the year (19), week number (05) and weekly identifier (a). In the example case (19w05a), the snapshot is first (a) snapshot of week 05 in 2019.
So, to play Java Edition 1.14 development versions, you need to first enable snapshots from the launcher settings and play profile "Latest Snapshot".