This is commonly suggested, and entirely useless.
What the ransomware groups do is put a time bomb on the computer, then leave it to trigger on a future condition. Your backup will backup the time bomb, and the second you restore it, it also goes boom. And therefore your backup is a perfect copy of your data but entirely useless.
Even if your backup does couple the data and compute together, if it's simply time based (not sure what other event you could use really, perhaps some pure probabilistic function), then it seems like you can just trick the environment that the time is something else to get back in.
The real underpinning issue is that this stuff breaks the state of the infrastructure and the business can't afford the downtime to go around and repair these issues.
If you have your infrastructure build out mostly automated, that automation is backed up, and critical data is backed up, then you can reasonably sidestep these issues (I supposed a real thorough breach might integrate the ransomware in this very automation system but it should be reasonable to root out). The other issue is of course if the intruders threaten to release private data (empkoyee and customer PII, financials, so on). There's also business integrity but that doesn't really seem to matter anymore.
But as for an event to use, what they can do is have the machine check a remote URI to see whether it should let the system run, and if it should then set itself up to lock things at a specified time. In order to restore that you need to have it starting on a network with networking to a system that has the attacker's private key to sign the request. This is not an environment that you are able to create.