Although both this forum and WeeDogLinux use phpBB, to register on WeeDogLinux I had to type in the answer to math equation displayed as a graphic image.
A year or so ago, I looked into setting up a blog with an associated forum. Its content was to be about politics and economics: likely to attract --if no one else


Both Forum Software use plugins. I don't recall which had a problem getting, I think, Recaptch3, to work: what google calls 'the no-captcha captcha". But, you know what I think about ceding more power to google in any event. I think recaptcha 2 is a table holding 9 images (3x3) where you have to click only and all those displaying of a particular item, e.g. cars. Wiak may have opted for the original captcha. On one forum a different mechanism was frequently recommended: requiring the typing of answers to a question.
I would think a question such as "What is the latest version number of radky's most recent application for customizing jwm?" would be easy for anyone actually interested in Puppy Linux to find and answer but impossible for bots. Weekly/frequent question changes would prevent valid answers from being share by bot-deployers.
Puppy Linux Discussion is far less likely to attract problems than the forum I was considering. Still, as your forum attracts more attention, some of that attention may be unwanted. Weedoglinux is an even more niche website that Puppy Linux Forum. You might consider asking Wiak why he decided on a spam filter; and why the one he selected.