I worry that attitudes towards BLM by both the OP and the commenters strongly correlate with reactions to this research, and that the ensuing discussion is going to be highly politicized. It might perhaps be more fruitful if parties on both sides instead try to make a series of explicit and verifiable predictions related to this research, such as whether it will be published in a peer reviewed journal without significant alterations, or whether a similar analysis on the effects of last summer’s historic wave of protests will yield similar results.

It's important to highlight this point from the article:

It’s worth noting that Campbell didn’t subject the homicide findings to the same battery of statistical tests as he did the police killings since they were not the main focus of his research. (He intends to do more research on how these protests affected crime rates.)

Just a word of caution before we jump to conclusions.

Also, I think the main findings from the research were extremely interesting in their own right; That the BLM protests were successful in achieving some of their intended aims. The effectiveness of protests are rarely assessed quantitatively so glad to see someone doing this work. It would be interesting (and probably extremely challenging) to do a cost-effectiveness estimate for BLM, considering you have to account for the counterfactual value of 350,000 people etc.

A new study, one of the first to make a rigorous academic attempt to answer that question, found that the protests have had a notable impact on police killings. For every 4,000 people who participated in a Black Lives Matter protest between 2014 and 2019, police killed one less person.

..

From 2014 to 2019, Campbell tracked more than 1,600 BLM protests across the country, largely in bigger cities, with nearly 350,000 protesters. His main finding is a 15 to 20 percent reduction in lethal use of force by police officers — roughly 300 fewer police homicides — in census places that saw BLM protests.

Did they check the sizes of the effects over time since the protest? If BLM achieves their aims and gets the reforms they want, killings by police might remain reduced (but perhaps less so), while policing would become less politicized and scrutinized, so become more effective again.

EDIT: Towards the end of the Vox article:

The good news is that even if Campbell’s finding about the increase in murders following BLM protests holds up to further scrutiny, the effect doesn’t appear to last for long. By year four, Campbell no longer observes a statistically significant increase in murders, indicating that whatever is going on with murders is hopefully not long term.

I think you should modify the title to include "in the short term", since the increase in non-police killings was temporary. It seems pretty plausible that killings are reduced overall in the longer term.

released his preliminary findings on the Social Science Research network as a preprint, meaning the study has yet to receive a formal peer review.

It’s worth noting that Campbell didn’t subject the homicide findings to the same battery of statistical tests as he did the police killings since they were not the main focus of his research.

I thought there had also been some cautionary tales learned in the last year about widely publicisng and discussing headline conclusions from preprint data without appropriate caveats. Apparently not.