I'm not exactly sure how you've spent so much time arguing in this thread and others without being able to answer that question. It never stopped is the answer.
1950s and 1960s shouldn't need much explanation, the black civil rights movement is in full effect.
https://www.history.com/topics/black-history/civil-rights-movement#section_1
Now, my understanding about systemic racism and law enforcement from the 1980s on has to be understood through the lens of the "tough on crime" mantra and the war on drugs that we see being pushed by Nixon, Reagan, Bush senior and Clinton.
The situation is that the black civil rights movement has won, the language of the law no longer discriminates against skin colour. It is no longer politically tenable for a candidate to talk about instituting explicitly racist policies. However, if there's a need for a civil rights movement in 1959-1968, do you think 1980s US is racism free?
Anecdotally, there has never been a time period in the US since then where there hasn't been a substantial backlash against police brutality against black Americans. Cities across America have a rich history of protests against this, it is verging on conspiracy theory to argue that all of these experiences don't add up to a larger picture.
There are many high profile cases surrounding law and race and the reason that they blow up is because there's a great deal of frustration surrounding this topic that is felt around the country. When you consider that only some decades prior you have the black civil rights movement, fighting against literal segregation, it becomes even more implausible that there wouldn't be issues between law enforcement and black Americans.
Statistics show that the war on crime has taken prison populations from around 200k in 1970 to 2.3 million now. It's important to understand exactly how the process around getting a conviction works. Firstly, let's deal with the idea of the "fair trial", cases generally don't go to trial.
https://tinyurl.com/ycmwzas6
That is important to remember when hearing about the statistics of the results of the war on drugs. Which is that there is extreme racial disproportionately in incarceration rates with black Americans constituting over 35% of the prison population while only being 12% of the population. There's a supposedly a 1/3 chance for a black male to have served time in prison but 1/5 have already served time. That's happening without trials for the most part.
When you get out of prison, you are something like a second class citizen. Not only did you lose years of your life but it's going to be harder to get residency, a job, social benefits, you may not be able to vote and so on.
Police brutality is a phenomenon that goes beyond racism but the US approach to crime exacerbates the issue. Again, I don't want to give narratives, anyone can give narratives but the war on drugs and petty crime ends up manifesting as a war on poverty. From the way police treat you that leads to the arrest and the way the state attorneys treat you in what kind of deal they offer or whether they are willing to drop charges or you can afford to go to trial or not.
That's just dealing with the statistics where we got to that stage, if 1/5 black males have been to prison, then how many black males are having unfavourable experiences with police? What is the impact on the affected communities?
When you combine the prevailing attitudes in the states with the statistics and the history, it paints the picture of systemic racism.
Now as for whether you can argue that this is just a huge mistake and that the US government never intended to treat the races differently, it is pretty much ridiculous perspective. Honestly, the results speak for themselves but there's a lot of evidence to support the racist undertones of the war on drugs and its origins. The way that the various drug epidemics were handled i.e compare oxycontin with heroin or cocaine vs crack demonstrate inconsistencies. When you look at the statistics, it paints a picture as well.
On top of that, you have infamous admissions those involved in campaigns for Nixon and Reagan being explicit in the intentions but there is some controversy surrounding that. See
https://en.wikipedia.org/wiki/John_Ehrlichman and I forget the name of the other guy but you can do your own research into this.
I think that one can reasonably prove that the US government has purposefully constructed the relevant laws in ways that they knew would disproportionately affect the races. You need to look at how the US governments handle politics, the major goal is getting the party re-elected and everything done takes this into account. The policies appeal to the racial undertones that have been present in the US and still are. Nonetheless, the result can't be argued to be racially neutral.
There's a lot of room for interpretation here but there's a level of inexcusable simplicity in thinking that because the government doesn't use language that targets race, they can't be racist. That laws that don't mention race can't be part of systemic racism. I encourage you to further your education on this vast topic, if you're going to be as involved as you have been in this discussion.