We should hide that field at all for IMAP, as it's just confusing. It only applies to (in reality) NNTP - https://searchfox.org/comm-central/rev/e1f037dd2e1292f1fbff005a4729b95412c2e625/mailnews/base/content/subscribe.js#148
Bug 1630286 Comment 1 Edit History
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We shouldn't show that field at all for IMAP, as it's just confusing. It only applies to (in reality) NNTP - https://searchfox.org/comm-central/rev/e1f037dd2e1292f1fbff005a4729b95412c2e625/mailnews/base/content/subscribe.js#148