They got rather heavy-handed in recent times, though: all icons are behind a registration wall and an anti-scraping ToS even if the author declared them to be e

They got rather heavy-handed in recent times, though: all icons are behind a registration wall and an anti-scraping ToS even if the author declared them to be e.g. CC-BY, so while the result is royalty-free I wouldn’t count is as open source. (The website doesn’t call it such, either, so there’s that.)