It would be nice if sleep support can be added to LMIC-node, but the problem is that it will have to work for all MCU families and all supported boards (where possible).
Unfortunately MCCI LMIC still does not provide any sleep support (hooks/functions) to work with LMIC.
Only when MCCI LMIC adds such support, will it be worthwhile to investigate if it is possible to add sleep support to LMIC-node.
For users who want to add sleep support to their solution, you can start with LMIC-node and then add (deep) sleep support to it yourself (but this is not further described here).
It is currently not possible to add sleep support in a generic manner that works for all MCU (boards) supported by LMIC-node. Different type of MCU will/may require a different implementation.
For discussions related to that subject please start a separate topic.