No dog in this particular fight as I don't care either way but to answer your question: I've seen plenty of examples involving TERFs or questioning transgender women in women's sports where the ban hammer was brought out.
Another example is being egalitarian instead of equitarian. Being strictly egalitarian gets you labeled racist pretty quickly in many online forums. Just this past week in my neighborhood Buy Nothing group someone put up an item and made it available only to BIPOC people. Some people called it out as discriminatory and antithetical to community-building. They got called racist and accused of white fragility and were both muted for two weeks. No one on the side of orthodoxy got muted.
In both of the racial examples, people are attempting to correct past injustices. BIPOC people never received a court settlement for the injustices against them. Politicians left it up to citizens on how to repair the damage, so to do nothing seems far short of adequate, but it's a fair debate about how far to go.
How many black people still alive today saw signs that said "No blacks allowed" back in the 60s? I guess your neighbor saw the Buy Nothing post as basically saying "No whites allowed".
I could see the original poster's intent being "I'm sorry BIPOC that you haven't been treated as fairly as I have, and I want to help". Calling that discriminatory is a bit harsh, but I'm sure that doesn't feel good to a non-BIPOC person who's struggling and could really benefit from the item.
I think there's a concerted effort since George Floyd to understand the plight of BIPOC people and help them feel safe and respected in the community, where they at times have felt the need to be invisible to be safe.
It's like doing a fund raiser for indigenous people, trying to acknowledge that a group has been wronged and trying to help that group feel respected again.
But it's a fine line for sure. At the point it becomes an anti-white thing rather than a pro-BIPOC thing, the conditions change dramatically.
We should point out too that white people did get reparations. They lost slaves and the government paid them. You can't claim to be a "strict egalitarian" if you support a system that treats people unequally.
Another example is being egalitarian instead of equitarian. Being strictly egalitarian gets you labeled racist pretty quickly in many online forums. Just this past week in my neighborhood Buy Nothing group someone put up an item and made it available only to BIPOC people. Some people called it out as discriminatory and antithetical to community-building. They got called racist and accused of white fragility and were both muted for two weeks. No one on the side of orthodoxy got muted.