keyboard accessible? are most html trees?

They definitely should be, for accessibility reasons and because it's not that difficult to support.

Here's an example of a tree widget that can be navigated by keyboard: https://www.jstree.com/docs/html/

I'm not a fan of javascript being used for the clicky handlers, even if the element being clicked is a natural clicky element like an anchor tag.

It's much better to rely on a toggle like input type=checkbox and CSS to visualize tree expand and collapse based on checkbox states than to install click handlers in javascript. I suspect you could make such an arrangement more accessible and keyboard navigable as well (via grouping.)

Javascript should only be used to dynamically load/unload subtrees on click in the case of especially huge or costly trees.

As a blind screen reader user, I rely on keyboard accessibility for navigating any type of tree. Now I know not to checkout your project as it would be of no use to me.

Most aren't. The good ones are.