Proxyunbannable is a userright granted to sysops by [[mw:Extension:AutoProxyBlock]]. But this extension is not used on WMF wikis, which instead use [[mw:Extension:TorBlock]], with torunblocked as equivalent. Torunblocked is not granted to sysop, so they cannot edit from tor. However, for some reason proxyunbannable appears in the listed rights of sysops, with the description "Bypass automatic blocks of proxies". This led to some confusion among users. Since it doesn't seem to have any use, it would be best to remove it.