I had this bad habit my mum would put chilli paste on my thumb And it worked she was consistent

It might be wise to see if you can work on reducing / stop this habit.
Some long term effects of thumb sucking beyond the age of 5-6 years include: Teeth being pushed around, which might cause an overbite or an underbite to form. A lisp being formed because constant thumb sucking can affect the jaw bone positioning. Germs from your thumb and surrounding areas affecting your body.
My eldest sucked her fingers till she was in Kindy. She developed a strong overbite and her jaw was positioned in a wrong way. At the age of 8yrs she got a jaw expander for half a year, followed by braces.

To be honest if it’s not affecting their speech or teeth I wouldn’t stress about it.
My son was 13 when he stopped sucking his thumb and the only reason he did was because he got a cold sore on his lip so he couldn’t suck it.