We also went through something similar, what helped us was giving a replacement comfort soft toy or fidget and gently reminding them each time rather than making it a big issue. It took time, but consistency worked. We also found positive praise and small rewards really encouraged progress over time consistently.

Breaking a thumb sucking habit is can be very hard indeed. Thumb sucking is a self soothing technique and you can think of it as a potable coping mechanism. You can’t just take this away. You really need the replace it with another soothing method which could be something like an object (like a teddy, a fidget or a chewable necklace)

My mum would have loved a solution. I sucked my thumb as a child, and I remember her saying that even as a teenager when I was sick, I would still do it. We tried the nail polish, hand washes and even gloves occasionally. The cotton gloves seemed to work best from memory.

I’d love to know the solution to this. My toddler is a big thumb sucker, always has been, and I get the feeling that it will be a hard habit to break. It’s still early days, but I’ll most likely be looking at ways to intervene and find a way to deter him

There is also a glove on the market called “anti thumb sucking glove” or “thumb sucking guard” that only covers the thumb or the thumb and pointy finger. You can order them online by Amazon, Temu, eBay, Ali express, Thumbsie.
My daughter sucked her fingers and developed quite an over bite and she had braces already at the age of 8

There’s a product on the market what you apply on the nails/fingers which should help.
I think in general it doesn’t help to forbid your child to suck her thumb, but rather distract her from it, limit it step by step and see if you can offer a replacing soothing technique. for example a fidget or squeezy toy or a chew necklace.