Webhooks rely on the underlying Helix API. So if you can’t use webhooks, just poll the API and track new followers yourself.
Also for an alerts system, I’d also high recommend a server so you can properly cache and de-dupe followers, otherwise if you’re just doing this all client-side (which is what I assume you are intended to do when you mention serverless, rather than serverless technologies such as AWS API Gateway/Lambda which would work perfectly fine with webhooks) you could easily get spammed by someone following/unfollowing, and it’s not always viable to maintain a proper cache to prevent that when it’s done client-side.