This release primarily exists so the default version is 1.9.10, but it also makes some changes to match the official setup's support for resizable GWT applications (and it includes a useful part of a currently-unmerged PR, libgdx/libgdx#5691 ).
There's not many breaking changes in libGDX 1.9.10, so it's unlikely but possible that projects using 1.9.9 would encounter breakage. There are several breaking changes from 1.9.8 to later versions, so if you're updating from further back you may encounter some issues. I made this Gist that you can use to replace SpriteBatch if the changes to Batch color handling between 1.9.8 and 1.9.9 (and persist into 1.9.10) are a problem; in the uncommon case where rendering sets the Batch color extremely often, especially using packed float colors, then that Gist is something to try because it will make the API match 1.9.8 and should perform better if colors are being packed into floats often.