I can confirm they do now charge separately for them. However the cost is very reasonable, around 5-10% of a full sandbox.
We were asked to confirm if we used ours, which we did confirm, but they took them away earlier this year despite that. We had no option to keep them for free.
The biggest issue is that if you want to keep the data on your developer account, you have to renew it while it is active, or within 30 days of it being unavailable, after which time NS will discard the data from it. If you don’t care about what is on them, then the timing of when you renew doesn’t matter.