What is the "escape sequence" for "Hayes Command Set"? Where was this used in the past? Where is it used today?

The Hayes escape sequence is +++ — three plus characters.

How it works: the Hayes "AT" command set distinguishes between data mode (the modem is passing your bytes transparently over the link) and command mode (the modem interprets your bytes as commands). To switch from data mode back to command mode without dropping the connection, you send +++. Crucially, it is protected by a guard time: there must be a period of silence (no characters) before and after the +++, and the three pluses must arrive close together. This guard-time requirement is what prevents an ordinary +++ appearing in your data stream from accidentally knocking the modem into command mode. After a successful escape, the modem responds OK and you can issue AT commands (e.g., ATH to hang up); ATO returns to data mode.

Where it was used in the past: the Hayes Smartmodem and the countless "Hayes-compatible" dial-up modems of the 1980s–1990s, which made the AT command set the de facto standard for controlling modems (dialing ATDT, answering, configuring, etc.).

Where it's used today: the same AT-command paradigm — including the +++ escape — lives on in modern communication modules: cellular/GSM/LTE modems, GPS, Bluetooth modules, and many serial-to-wireless radios are still controlled with AT commands, and many implement the +++ guard-timed escape to flip between transparent data passthrough and command/configuration mode.

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