It is assumed knowledge however you don’t actually end up using it anywhere I believe. It may be used in showing proofs for geometrical concepts in the background info in your textbook (depending on which one you use, I know Cambridge uses similar triangles for background info a couple times).
I don't agree that similar triangles are not used, they can come up in some topics.
For example, consider a max / min problem like:
ABC is triangle with AB = 12 cm, AC = 20 cm, and a right angle at B. X, Y, and Z are points on AB, BC, and AC (respectively) such that BXYZ is a rectangle. Find the maximum possible area for this rectangle.
There are ways to skip the similar triangles here, but many students would use them in this applications of calculus question. In MX1 and MX2 papers, it is easy to construct a question where including a similarity proof is difficult to avoid.
"Assumed Knowledge" will not necessarily be included in a textbook or tested with a direct question, but it provides tools that are vital for answering exam questions. Most text books will not cover solving quadratic equations, for example, but a question like "sketch the curve y = x
3 + 2x
2 - x - 2 showing all stationary points and show that the x co-ordinate of the point of inflexion is at the average of the x co-ordinates of the turning points" is going to face you with a quadratic.