Turns out itch-setup couldn't go through SOCKS5 proxy. I solved it by using a tool named SocksCap64.
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For ppl who encountered the same problem:
Download it at https://sourceforge.net/projects/sockscap64/
Go to Proxy, add 127.0.0.1 at port 1080 (which is your shadowsocks client).
Go to the plus sign beside Apps, add an executable file, which's itch-setup.
Run itch-setup in proxy - problem solved.