If you remember sucking your thumb, it was probably well past the age of 2.
Some children suck for comfort and others do not. There is probably a genetic link, but for now it's just a fact of life.
Whenever the issue is about something the child is doing to soothe herself, it's never helpful to look at the behaviour that's soothing, but the underlying need that requires the soothing in the first place. Falling asleep is anxiety time for most small children, as they're losing conscious control of what is happening to them, and, I think, getting younger and younger (developmentally) making them respond like much younger children by the time they're nearly asleep.
If you hope to eliminate the need for thumb sucking (without, I'm assuming, replacing it with something else you'd like to eliminate before she's ready next year--like a pacifier or a cuddle buddy) you need to help her feel secure and relaxed going into sleep. For small children who cannot be reasoned with about fears, that pretty much means being with her, patiently and calmly, throughout the falling asleep period. Yes, every night. As an estimate, probably for the next 2 years, maybe less.
Essentially, you get to choose: self-soothing (in whatever way the child finds) or parent-soothing. None of the above is not an option, because if you're not right there for them, you're also not there to stop them sucking things, pulling things, banging things, rocking...whatever they find to allay their anxiety with.