Marathons, major festive carnivals/processions, easily cross this limit. I've had more failures than success creating such polygons and there are times I give up and involve the whole city straight, which is no where near accurate.

Have ran up agains the 20km limit countless of times this past year - from rapid transit expansions that are 16 km in length (easily exceeding 20 square kilometres) to also crisis polygons.
With the rapid changes of crisis of say wildfires, its very normal for these to be hundreds/thousand sq km.

Currently we're setting these traffic events to the province/state area, but they overlap, we cannot relay jurisdictional information related to regional websites about detours, etc
And without the polygon, it doesn't show up on the trip overview in the app - which for a crisis is kind of important.

With regards to crisis events, I'm also going to plug my Suggestion of the need to have ability to view/filter shelter pins from the sidebar (similar to how we can do with events/closures)
https://waze.uservoice.com/forums/59225-map-editor-suggestions/suggestions/51641359-list-and-filters-for-crisis-shelters-in-side-pan

We attempt to use unplanned disruption major traffic events in alerting Wazers to wildfire evacuation areas. Needless to say, in the western United States especially, 20km polygon limit just doesn't cut it. And much of the land is in unincorporated areas, so using a city name and state won't work either.