Long, long ago in earlier version of Moneydance there was a feature to archive data. This created more problem than it solved, so it was killed off by the developer.
in 2019 version transaction are recorded as if in a journal. Some people by deleting data such as transaction hoped to reduce the data set size but the delete of transactions is recorded and your data set simply gets larger.
The only safe way is to start a new data set and keep the old data set so you can open it any time you need to.