I have a popular web based ascii text generator (https://patorjk.com/software/taag), it uses FIGlet driver that I wrote (https://github.com/patorjk/figlet.js/). Many of the other ones out there also use their own drivers (I've seen some written in php and python). There are also different types of ASCII text fonts out there other than FIGlet (ex: my app compiles fonts that were made in the AOL days in the Arial font - which is interesting because it's a non-monospaced font).

FIGlet is the most popular spec for these types of fonts though (it's an app and a spec). The mailing list hasn't been active in a decade, but people used to submit fonts there. It was cool to see what people made.