The NeoLemmix 2KB level format (the one based on the DOS format but with the unused bytes repurposed) is no longer supported from V1.33n of the player. Actual DOS / traditional Lemmix levels continue to be supported; and this old NeoLemmix format will for now continue to be supported by the editor. NeoLemmix features in these levels that were back-ported to traditional Lemmix (ie: custom music track selection and oddtabling) will also continue to be supported.
I don't think any levels still exist in this format, but if you have any, please upgrade them (by loading and saving in the editor, or re-dumping them from a player via the "Dump All Levels" option) to a newer format; the new variable-sized format is ideal, but the 10KB format is also remaining supported for the forseeable future.
The most obvious way to identify such a level would be if its filesize is 2KB rather than 10KB. Otherwise, assuming they haven't been upgraded since, levels are likely to be in this format if:
> They were made with NeoLemEdit
> They were made with a NeoLemmix Editor version prior to V1.16n-B
> They were made with a NeoLemmix Editor version prior to V1.25n-A and the "Extended Format" option was not selected
If they've been made or edited with NeoLemmix Editor V1.25n-A or newer, made or edited with NeoLemmix Editor V1.16n-B or newer and the "Extended Format" option was used, or mass-dumped from a NeoLemmix player V1.16n-B or later, they'll be in the 10KB format. This covers all NeoLemmix levels I've ever seen released, but in case anyone has any that they haven't released but want to keep, I'm making sure this is well known. As mentioned - the editor will continue to support such levels for now.