There's been a fair profundity of WiFi modems knocking about - e.g. WiFi232, WiModem etc. These are used for connecting to online BBS's or even running them (e.g. https://www.telnetbbsguide.com/bbs/a-80s-apple-ii-bbs/). They all use the ESP8266 module and some form of RS232 level shifter. None of them are open source even if they use some open source firmware.
I've just released some open source hardware which works with open source firmware:
It implements all of the RS232 signals (DSR/DTR/DCD/RING/RTC/CTS/RX/TX) and, being open source, anyone can make them (and sell them if they so desire).
The project is available here: https://github.com/stardot/esp8266_modem/releases
Build instructions and BOM are in the Wiki. To programme it you just use the Arduino IDE and a cheap usb-serial adapter.
I've also been working on tweaking the Zimodem firmware (more fully featured) to work with the hardware here:
https://github.com/drdpj/Zimodem - currently it uses software implemented RTS/CTS, but I intend to adjust it to use the built in hardware flow control the ESP8266 provides.
I'm not intending to make these, but I'm happy to help anyone who wants to!