Lemmy issue #2922 is where it was removed 2 weeks ago due to its effectiveness being limited, Lemmy issue #3200 is the issue where it is suggested to have it return. Repo maintainers are somewhat perceptive to reverting it but would be more willing to accept a full implementation of graphical CAPTCHA over the previous version.
Alternative links: https://lemmy.ca/post/793011, https://beehaw.org/post/667199. (Note that if you view from a defed instance with Beehaw you will only see a handful of comments).