Post implants 10+ years - Developed what feels like little tiny pebbles right underneath the skin. What could these be?

That is an interesting question.... I wish you had posted a picture. It is very hard to know what you are seeing and feeling, without seeing and feeling.
I'm guessing that you are referring to Montgomery Glands. Most women have 10-40 little bumps of their areola, around their nipple. These are tiny glands beneath the skin, some are larger than others that serve 2 purposes. 1) they secrete oil to keep the areolar skin soft.
2) They also release a very faint odor that draws a newborn to the mother nipple, helping them to latch on. The more Montgomery Glands, the faster a newborn will latch on.
I do not know if this is what you are feeling in your situation. It may be, but it is still worth being checked by a breast specialist. If it is not this, then maybe a biopsy might be the next option.
Good luck to you

You might be feeling your Montgomery glands, which are normal in the areola, or may have some areas of scar tissue or even tiny cysts. I would suggest going to your doctor and being examined.

You may be feeling your Montgomery glands which are normal. If not you may need an imaging study that is more sensitive to determine the nature of the pebble-like masses. An MRI should give you more information. I encourage you to discuss the situation with your surgeon or PCP or whoever you see for breast health.