Every six months for the last 24 or 25 years (the web pages came after the early releases so a bit of uncertainty there. Explore for yourself at https://cvsweb.openbsd.org/www/)
And even vi gets fixes these days (Fixed handling of escaped backslashes in vi(1) ex_range).
Nice. I ran into an nvi bug years ago... if ":set numbers" and ":set noleftright" and then loaded a file of more than several kilobytes it would go into a malloc loop and segfault. We tracked it down and submitted a patch (this was to FreeBSD) and was told patches to src/contrib were not accepted (nvi was still "owned" by sleepycat). Good to know that at a certain point, function can overtake form!
And even vi gets fixes these days (Fixed handling of escaped backslashes in vi(1) ex_range).