There's tons and tons of anecdotal evidence for both short-term and long-term abstinence (Reddit is full of them. My own experiences and those of my friends uphold the same). Whether Placebo is a big part or not? I think that question is immaterial - since "if it works, it works".

For long(er)-term retention, actually "Pure Retention" (aka abstinence not just from seminal ejaculation but Lust itself), eastern (mainly Vedic) scriptures are chock-full of support for the same. The anecdotal experiences of the same are also immense (YouTube has many such genuine ones - minus the NoFap fanatics' drama). The research "evidence" for these cases is lacking because the samples are few and hard to find! How many do you know who've retained for 6-months+?

And this is the realm of the Subtle - just physical retention without mental retention/abstinence doesn't count for much. There's nothing "magical" about these beliefs - without effective transmutation and solid inner/spiritual work. the "emotional instability" part is bound to arise. I made similiar mistakes with retention in the past.

For shorter-term retention (mainly with abstinence from p*rn), there's quite some research in support. Far too many articles and videos of/on the same already out there, so I won't link any specifically.

I have zero gripes against healthy sex and masturbation (without fantasy or p*rn) in moderation - but beating the meat to p*rn multiples times a day or week? That's where most of the problems arise.

I don't doubt your expertise as a neuroscientist - but I'm sure you agree with much of the p*rn-abstinence benefits - Andrew Huberman has talked about the same a lot too if I'm not wrong.

Much of NoFap/Retention is mainly about quitting p*rn - the physical abstinence parts/benefits come much later.