I have not noticed any racist stuff on nextdoor, I think the leads in the groups I have participated in would remove these kinds of messages and their participants.
I have been/am involved in three communities, all in the southeast:
1. An upper class neighborhood in a city where whites are a (slim) minority
2. A working class neighborhood in the same city where I seemed to be the only white guy
3. A rural neighborhood where almost everyone is white
I am surprised that people would write such things today, when the perils of doing so with your own name are widely understood.
The penalty for making publicly racist statements is decreasing, IMO; people are still being picked off at the leading edge when they really do go unambiguously too far, but there's an entire industry of thinly disguised racist news media.
Are you speaking up when you see these examples of racism?
(Seriously with the downvotes?)