Yes, your account would be gone.

I know that its pretty normal for Mastodon server operators to publicly commit themselves to anouncing that they are going to shut down, so users can take action accordingly (Was it 90 days, not sure?).

Additionally, Mastodon has a functionality that lets you transfer your account to a new server you choose, while keeping your followers and followings.

Lemmy doesn't have a similar functionality. I suppose this is something in the works. Ideally you would take your community subscribtions with you (instead of followers/-ings).