Question: Does LiftWing have a way of categorizing whole WikiProjects under the topical categories too, or is it only the articles within the WikiProject? I'm asking because we're thinking of a project to recommend events to editors based on their edits/interests -- but can we also do the same for WikiProjects?

Answer: yes and no: we hand-built a list a while ago for this for English Wikipedia based on their WikiProject council: https://github.com/wikimedia/wikitax/blob/master/datasets/wikiproject_taxonomy.halfak_20191202.yaml. that list is old and I've made some changes recently but I forget in this moment where I put them. but there are other ways too. for example:
fetching a sample of articles from a wikiproject and seeing what the most common topics are per LiftWing and using that to categorize.I could also imagine for your use-case, just grabbing a set of similar articles to the ones that the editor has updated and checking the most common wikiprojects on them. that's what I do for the list-building tool if you scroll down to the bottom of the page below the results table for a query. Might be a way to use Wikidata. I think I looked at this a while ago around main-topic of subject's-main-category properties.

caveat with all of this is that only a few languages have a nice structured way of tracking which articles are part of which wikiproject: https://extloc.toolforge.org/extensions/PageAssessments (edited)