I don't think there's any solid evidence of disproportional use of of one ethnic group by another. You can certainly find individuals who thoughtlessly or deliberately do that but generalizing that to some random population (eg of office workers) is likely wrong.
I think that's just some form of confirmation bias you've formed, as I've never noticed that. I will say most of my offices have been multi-cultural, but even then, I've been on and led teams of white-only males that never succumbed to even posting these gifs. I guess guilting people into censoring their choices is a point of action, but I think the entire process should just be faded out. Slack admins don't need to allow these plugins. Theres nothing stopping people from literally going to giphy.com or whatever, but that's not why it's popular -- and it's definitely not why FB is shelling 400 million for it.
Digital blackface is not my idea, nor is it something I noticed. (I am white.) Someone else explained it to me. I was probably a bit skeptical that a funny GIF of Oprah could be linked to Jim Crow, but in the historical context of blackface there are some unavoidable similarities.
The term to Google here is “digital blackface”.
If you bring this to more people’s attention my guess is that reaction GIFs will become less prevalent in your office.