Been perusing the code - it seems like ES is only used to log some queue metrics (how long the queue was and the avg wait time) and some event metrics. So I would not consider that information that needs to be restored.
On that note task_bll.py creates an events_es instance but nothing ever seems to use it. Same for the redis instance.

Hi @<1585441130525233152:profile|TrickyGoose45> , ES is actually much more instrumental to the ClearML Server, as it's used to index all reported metrics (experiments as well), so backing it up is absolutely required

Redis is indeed transient and backing it up is not mandatory

Ok thanks, is it sufficient to backup mongo and elastic independently or do they need to be backed up in a synchronized fashion?

As long as it's a backup from the same time, it should be OK