Threats with the goal to prevent publication are incredibly common.
Following up on the threat is much less common, and the best way to prevent that (IMO) is to remove the motivation to do so: Once the vuln is public and further threats can not prevent the publication, just draw more negative attention to the company, the company has much fewer incentives to threaten or follow up on threats already made.
It's not a guarantee, you can always hit a vindicative and stupid business owner, but usually publishing in response to threats isn't just the right thing to do (to discourage such attempts) but also the smart thing to do (to protect yourself).
Following up on the threat is much less common, and the best way to prevent that (IMO) is to remove the motivation to do so: Once the vuln is public and further threats can not prevent the publication, just draw more negative attention to the company, the company has much fewer incentives to threaten or follow up on threats already made.
It's not a guarantee, you can always hit a vindicative and stupid business owner, but usually publishing in response to threats isn't just the right thing to do (to discourage such attempts) but also the smart thing to do (to protect yourself).