There is no way to disable bold for "true" and "false" keywords.

[devshmsec]

I have set all styles to { bold = false } but still "true" and "false" are left out. How to remove boldness from these keywords?

functionStyle = { bold = false },
commentStyle = { italic = false, bold = false },
keywordStyle = { italic = false, bold = false },
statementStyle = { bold = false },
typeStyle = { italic = true, bold = false },

[rebelot]

I can't reproduce. If you are using compilation feature remember to reload neovim and use the command :KanagawaCompile

[devshmsec]

I am not using compilation feature. I also tried different terminals but still have this issue.

[rebelot]

from your screenshot I can't see any bold statement or italic comment.. use :hi Comment or :hi Statement to make sure

[devshmsec]

I don't understand what are you saying. I disable every style with { italic = false, bold = false }, that's why there are no bold statement and italic comment. My problem is there is still bold decoration on "true" and "false" keyword.

[devshmsec]

finally fixed the situation in overrides option.

overrides = function()
    return {
        -- setting boolean decoration bold to false
        Boolean = { bold = false },
    }
end,

Thanks to u/zanza19 from neovim subreddit, he pointed out that problem is in this file.
lua/kanagawa/highlights/syntax.lua

--  Boolean	a boolean constant: TRUE, false
Boolean = { fg = theme.syn.constant, bold = true },

There should be type for booleans in config.