I've heard that too, but I think it would take a decent developer 1 hour to make a first pass at such a thing. It doesn't have to be complicated nor perfect. I think it should not require images nor javascript. It should probably be a server-side LUA script that caches the puzzle and answer.

I found a few starter ideas [1][2] and concepts [3] but I would prefer to use something like figlet vs gd generated images. Figlet or something like it should be much lighter weight. I just have to find one that is readable on cell phones.

[1] - https://github.com/lua-programming/lua-captcha

[2] - https://github.com/mrDoctorWho/lua-captcha

[3] - https://nedbatchelder.com/text/stopbots.html