The server version is commercial software, the price to buy a perpetual license to use the version you bought (and updates for one year) is listed here https://server.manager.io/buy So clearly the server version is not free.
After having paid for a perpetual licence to use Manager server, you can can continue to use that software (and releases up to 1 year) for no further charge. So you can say, after you have purchased a perpetual licence, that version can be used indefinitely for free.
The server version can also be evaluated for free. It uses an honesty system to enforce this. If you are honest you transition from evaluation to use as soon as you commit to put you financial data into the software and use the reports it generates.
Archive versions are available here https://github.com/Manager-io/Manager/releases but these are not archived indefinitely.
The recommended solution is to use the free desktop Manager to open the backup of your data file.
Your Manager data file can be imported into any of the newer versions of Manager (Server, Cloud or Desktop). Very old data files can be imported into the current Manager versions. The commercial terms to do this in the future will probably be similar to what is offered now ie doing so via the desktop version is free.
If you want to restart using the cloud version then you need to renew your server key here https://server.manager.io/renew the price has been about half the new licence cost. Again it is enforced via an honesty system.
Clearly an honesty system can be exploited by dishonest users. Doing so will eventually result in replacement of the honesty system with strict enforcement, or closure of NG Software. As a result the practice annoys me, but perhaps it’s easier to embrace it.