Jess, it will be in google SERPS for awhile.
Last I checked, enabling “disable subdomain indexing” also just sets the robots, txt, which means if Google has already indexed your site it, those pages will basically hang around forever.
You might be able to request a removal through GSC if you can validate site ownership through a META tag inclusion.
Another approach I use is to have a script that will add a noindex meta only to the webflow.io site.
You also may be able to just “kill” it, by changing the shortname of your site. Then the old webflow.io site will be gone and Webflow will eventually de-index it.
The hosted domain of brightsidekc.com is fully published.
I was publishing the subdomain in parallel for a bit too but had stopped doing just the other day.