I tried, and all I found was some hearsay. Ethical concerns were raised by many publications without any specifics. Some mention the issue is building on top of hyprland, which (oh god, no) has some ethical issues.
DHH made a blog post in which he implied (but did not state) that the UK isnt white enough anymore. Its gross and racist, but as far as I know this is all there is to the current controversy around him.
IMO, the implication is quite weak. The only place I found where the author mentions ethnicity, is when he refers to Pakistani rape gangs, and I assume, the choice if words is influenced by the article he links (can't verify, as the link to the article is broken).
Otherwise, this seems to me like an antiimmigration position, but I couldn't spot any racial or ethnical prejudice.
I couldn't find anything specific.
Would appreciate a link to any based explanation