"Data locked up in some software" in practice means strictly SaaS with cloud storage.

If you have the data locally, you can always migrate it - whether manually, through the software that made it (e.g. if the app isn't supporting your new OS, then run it in a VM with an older OS to export to a more forward-compatible format), or through third-party converters. If enough people are in this position, someone will write a solution.

If your data resides in the cloud, however, it's up to the vendor whether or not you'll be able to access it or export it, and how much of it. If they're just sunsetting the product, you probably, maybe, will be able to get some data back (which is almost definitely in some unknown ad-hoc format, quite likely a database dump in form of JSON, so you'll need a converter anyway). If they terminate your account because of $random reason, you won't even have a chance.

Or, to borrow a cliché, "not your files, not your data".

> or through third-party converters.

Assuming those exist.

They tend to for any software with non-negligible user base. The more so if those users suddenly all need one. Worst case, you can make one yourself. But none of that is even a possibility if the data is locked away in someone else's cloud.