This looks very interesting!
However, unfortunately the game doesn't track when a sector was created... you can find out if a sector was "modified" by having a look into the "sectors" table in the Chunk.db (if a sector is stored there, it means that it's considered "modified". All necessary chunk data is stored in the "info" column, so if the algorithm changes, it's not affected by this.
If you want to recreate a multiplayer world in singleplayer, you will basically need the Chunks.db (or at least the content from the "sectors" table). If you're using the Plugin API, you could extract the sector data through the WorldDatabase (i.e. WorldDatase.executeQuery()).
But I don't think there is really a way to find out if a sector was created before or after the biomes update... but on the other hand, why do you need this information specifically?
I opened table sectors from chunks.db before cave update and i got all coordinates of them ( x, z) ( x on horizontally growing to east, z on vertically growing to north - Correctly?) .
I found on world 30 sectors saved which i marked on map with green. Other are sectors starting to be discovered by players. (Blue ocean, yellow arid, light blue Cold.
PS: I got a report one sector saved was regenerated after cave update (i can't confirm myself because i don't have raw data from that sector), even if sector was found sectors table. Its possible to sector regenerated on this case?