Well, no - this data could be relevant ten years from now, if they’re accused of allowing an underage model after the fact. They need this data to protect themselves, and they need to have it accessible for the rest of their lives.

This is quite incorrect. Most jurisdictions have record retention policies that explicitly state that you're allowed to dispose records after a certain period of time (usually based on record type). After that, even if the record would be relevant to a case, you cannot be held liable for having disposed of it.

source: i work in this field

Isn't there a better solution involving offline storage? Keeping the data online is likely exposing it to insider threats as well.

Isn't there a statuate of limitations?

There are many, but capital offenses have no limitation, and there are many potential capital offenses. However, that shouldn't really be an issue here, because any business can commit a capital offense, but we don't require every business to record every bit of information about everything they do in perpetuity just because they might commit a capital offense one day. Innocent until proven guilty.

I think that there is a limit on the required period of record retention but I keep failing to find it.

But I don't think it is short, and it at least extends as long as you are selling the material to which it relates, since it is expressly illegal to sell depictions without the records.

If the data is to protect yourself against criminal accusations, there's not really a time limit after which data can be safely purged.

They could be accused many years after the fact.

Often the law that stipulates you need the record, also stipulates how long you need it. Good data hygiene is to retain in it for as long as necessary, and no longer.

It does not. They use old footage of models indefinitely and can be accused at any time of distributing such materials even after they’ve stopped distributing it when clients are found in possession.