The official changelog is:
Using #tags to keep your to-dos perfectly organized across projects? Then you’ll be happy to know that we now support #tags in notes, as well as the in the to-dos title in detail view.
Here's the new version 1.38.1808 (up from v1.34.11781) in action:
Adding a hash tag in the notes field here immediately gets it highlighted and hyperlinked (in blue). Tapping on this, or (at any point in the interface) searching for even part of the hashtag, brings up all matches for that hashtag, across all to-dos in all lists.
You can grab Microsoft To-Do here in the Store, if you haven't already. As mentioned before, this is shaping up to be an attractive and pretty powerful tool/service, syncing on multiple devices and platforms, and I'll be diving into more depth with it in due course.