According to user223149 while attempting to reply to this question,
Got this answer from support:
Our app Handcent Next SMS has been misidentified as malware by Google, we are in talks with Google to get our app back to play store. In the meantime, please keep our app and do not uninstall. You should still have all your data when it returns to normal. You can use the stock app for now. Don't worry, we are trying our best to restore our service.
This was later confirmed on the official Handcent website, Twitter account, and Facebook Page,
Our app Handcent Next SMS has been misidentified as malware by Google, we are in talks with Google to get our app back to play store. In the meantime, please keep our app and do not uninstall. You should still have all your data when it returns to normal. You can use the stock app for now. Don't worry, we are trying our best to restore our service. You should be able to see our back in a few days. We apologize for the inconvenience, and will continue to provide a good user experience in the future.
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For those who want a temporary solution, please contact us at help@handcent.com
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User WildBSinTX on Reddit sent an email to Handcent support and got a reply,
I got a reply this morning with an explanation and an attachment with v9.9.9.
*Security patch to fix a potential threat
*Other fixes to make sure it's in a good safe state
I installed it, and so far, so good.
On their main page, they say, "For those who want a temporary solution, please contact us at help@handcent.com". I'm sure they will email the attachment to you, too. Keep an eye on www.handcent.com.