We do not want to have lots of outdated versions on the download page. Therefore, only old versions that could be useful in some special cases - like 6.22 - are kept.

The MDL5 format is still supported by 6.50 - at least it's supposed to. If you've started a project with 6.31 it might make sense to finish it with that version, but in that case you normally have 6.31 already, and won't need an update.