Apple Watch Series 9 and Ultra 2 Gaining New 'Double Tap' Gesture Control Feature

The finger tap thing isn't new. Perhaps the trigger of some specific functions are new, but in the current WatchOS you can use Assistive Touch to customize many different gestures using a finger tap(s) or even a fist clench.

It almost seemed like Apple was about to make an entire event out of this one feature. ?

This "new" feature has been available since watchOS 8.3. Apple even did a video on it themselves, explaining what it does and how to set it up.

So Apple will be removing gesture controls for Series 8 and older if you upgrade to watchOS 10?

Don't be fooled - this is already on your watch under Accessibility.

The thing is most apple watches can already do this as an accessibility feature. It was almost comical this got such a long segment in the keynote.

It didn't gain Jack. This has been available under Accessibility since WatchOS 8.
Accessibility -> Assistive Touch -> Hand Gestures and then scroll down to Customize Gestures. Set Double Pinch to Tap and you can do everything shown in that promo right now. Or you can really get fancy and customize all the options like clenching your fist to go forward and back in menus.
I've been using these settings to control my watch during workouts for a couple years now, which is evidently how long it took Apple to figure out it had a useful feature it should promote, especially if it helps them sell a yawn of a new product that 100% isn't required to use it.