IMAP does support keywords, which are basically tags. Probably some good IMAP-Client supports this?
There is also JMAP as a modern alternative to IMAP in development. Fastmail released a beta-interface for this, which also includes tag-support AFAIK.
And finally, Outlook surprisingly seems to support tagging, but they call it categories(?). It's not obvious at first if you only know traditional categories.
I have used Thunderbird and Evolution mainly. They cannot really do that afaik.
> Probably some good IMAP-Client supports this? The Gmail clients do that (at least the last version's I remember 3 years ago). Haven't seen any other FOSS that does so too. That is what made going away from Gmail kind of "hard".
Currently I'm using K9 on Android and the web client by mailbox.org based on OX AppSuite. Both are ok, but usability could be better.
In any case, best of luck with it!
Thunderbird's tags are a different concept from folders on IMAP.
As I remember the way GMail is using IMAP keywords is considered "broken". They just do it wrong and make it impossible to play nice with alternate clients on gmail-accounts.