First you might want to read: Bonding Curve - ELI5
TL;DR: Bonding Curve replaces them
Essentially, Bonding Curves allow to your token to be traded through a smart contract. Removing the need for a traditional exchange, although nothing stops a traditional exchange to list your token. It is the same mechanism that on-chain decentralized exchange UniSwap and Bancor use to operate.
ICO platforms also become irrelevant in the sense that everything happens through your Bonding Curve. Augmented Bonding Curves can have a Hatch Phase which shares similar goals to an ICO: “During the Hatch Phase, initial contributors (i.e. founding members of the organization, devoted contributors, initial investors or “Hatchers”) participate in a hatch sale. The point of this phase is to gather contributors and pool capital” https://medium.com/giveth/deep-dive-augmented-bonding-curves-3f1f7c1fa751
Feeling lost? you might want to start reading here Finance & Token Engineering category - START HERE