[amaccuish]

The problem is that in IMAP the concept of labels doesn't exist. Thus they are implemented using folders, which is a concept in IMAP. But that issue is, a message is then in multiple folders (to reflect that it has multiple labels), but in the way this happens in IMAP, they are different messages, even if the content is the same. And thus I can imagine, automated programs, not aware of this, may for example see 2 invoices (because the email has 2 labels) rather than 1.

[k_g_b_]

IMAP has a concept of flags/keywords (https://datatracker.ietf.org/doc/html/rfc9051 2.3.2) and there's a whole bunch of servers and clients that support using it in a similar way as labels - Thunderbird, Zimbra, dovecot, etc.

It's not a feature originally built for labelling like Gmail, so it has a few complexities ("Servers MAY permit the client to define new keywords") and enough clients (especially mobile) that don't support it - but they could just like Thunderbird or some webmail clients.

I've used it basically exactly like Gmail labels for years on TB on desktop.