That is not possible and never was. What you could do, and still can within certain limits, is open older versions of a data file with newer versions of the program. Thus, the backwards compatibility extended to older versions of data files, not older versions of the program opening more recent data files.
The reason is that, as features are added, the database structure changes to accommodate new variables. Newer program versions include scripts to update older data files. But it is obviously impossible to go the other direction, because older conversion scripts did not envision later developments.
The limitation I mentioned came in version 20.9.89. By that time, the overall structure of the database had been converted to SQLite. For a long time previously, the conversion scripts would update older data files to SQLite. But by 20.9.89, the developer decided most users would have already updated and be operating in the SQLite environment, so that portion of the conversion script was eliminated. Therefore, really old data files must be updated in two stages, first to 20.9.89 or something a little earlier, then to the latest version.