He expressed extremely hateful views on Caucasians, gays, immigrants, and Jews in his blog posts and videos around 2010.
He pushed "the great replacement" conspiracy theory right in 2017 saying that Putin conducts Islamization of Russia (his article on why Trump is the opposite of Putin).
All these cases make him controversial for the modern world where people are "cancelled" for much less.
If you followed him more than 10 years, you would be aware of his extremely hateful comments against all kids of minorities in his blog.
For example, his proposals to kick all "rats" (Georgians) out of Russia and bomb them[1]. That's just one example out of many.
[1] https://navalny.livejournal.com/274456.html