Some years ago, I was playing on a world with some friends, when all of the sudden the server started with a new world upon starting the server. I got very frustrated and stopped using Aternos for a while. About a year later I came back and after encountering more problems, I complained in a german post on this forum. Official Aternos staff told me that, as at that time, the first world deletion was a year ago, this problem should not occur again. I was still hesitant to use Aternos. Skip forward to about a week ago. I checked out Aternos again and, after noticing how much the UI and capabilities of Aternos improved, I regained my trust in Aternos and created a server. (With the mod Conquest Reforged) We played on it for about a week (probably not more than 40h of play time). Just now, I started the server and found out that the world has been deleted. Not only that but it has been entirely reset. All mods deleted. White-list cleared. OPs reset. I suspect it might have happened due to me exploring the world and generating new chunks the last time we were playing on the server, however I do not know for certain. I did NOT receive a warning. Now, yes there is a fault on my part. I did not keep any backups of the world, I should have done that, but that does not make it any less wrong for this server hoster to be deleting worlds and resetting servers. WITHOUT any warning.
At this point, I am fairly sure I won't use Aternos again. Don't get me wrong. I like what this team is striving for, and I would love to see it flourish and become great. But with issues like these (lasting years, persisting even though they are supposedly gone), I doubt it will.
I hope you fix this issue ASAP. I wish other players the best of luck as to not get the worlds and servers they put hours of effort into get deleted.
EDIT: As someone commented, this could've been caused by a network issue crashing all servers and deleting their worlds, but as my server never crashed nor did I lose connection, this is very likely not the reason for my world being deleted. (Assuming that this issue only affected online servers ofc).