Hmm, this seems to be because of Google's new policy for verifying extensions. The domain (modernhn.com) hasn't been used for an extension before, so it needs to be around for a few months with a good track record to be verified. Their page [1] says:
> For new developers, it will take at least a few months of respecting these conditions to qualify.
Modern for Wikipedia [2] already meets those standards, and has the relevant badges, so I guess it'll just take some time for those to be approved and for the "Enhanced Safe Browsing" warning to be removed.
[1] https://github.com/glench/ExtPay
[2] https://news.ycombinator.com/user?id=glench