(Side note: It's been a few years since I looked through the specs and maybe I missed something)
IMAP flags are sadly severely lacking in what they can represent. The character set isn't even enough for unmodified UTF-7 (Punycode could work, but only for one-word tags) which renders them useless for user-chosen labels so few mail clients actually bother supporting them in that way. Thunderbird uses \label1 through \label5 but the names that are displayed in the UI are stored on each computer separately so you'd have to set up the labels you use on each machine (which kinda defies the purpose of storing that on the server). And short of using a »label configuration mail« on the server I don't see an easy way of supporting them completely server-side.
I'd love to completely forego folders and use tags/labels/flags/whatever they're called exclusively but last I looked IMAP wasn't able to support that.
IMAP flags are sadly severely lacking in what they can represent. The character set isn't even enough for unmodified UTF-7 (Punycode could work, but only for one-word tags) which renders them useless for user-chosen labels so few mail clients actually bother supporting them in that way. Thunderbird uses \label1 through \label5 but the names that are displayed in the UI are stored on each computer separately so you'd have to set up the labels you use on each machine (which kinda defies the purpose of storing that on the server). And short of using a »label configuration mail« on the server I don't see an easy way of supporting them completely server-side.
I'd love to completely forego folders and use tags/labels/flags/whatever they're called exclusively but last I looked IMAP wasn't able to support that.