The basic core functions of WP fail2ban have always worked with IPv6, more by luck than design, but they did work. However, none of the more advanced features (e.g. trusted proxies) or Premium features did.

That's all changed with v5: every part of WP fail2ban now fully supports IPv6.

One of the biggest challenges was performance. There are many existing libraries that abstract away the problems of dealing with 128-bit IPv6 addresses on a 64-bit (or even 32-bit) machine, but in testing they all added an unacceptable performance penalty.

There will be another post about this, but the spoiler is that v5 is no slower for short IP lists (e.g. one trusted proxy) and potentially much faster for larger lists (e.g. Cloudflare proxies).

Both the main plugin and the Blocklist add-on needed to be modified concurrently, which caused a delay to the new Cloudflare add-on one particularly observant user spotted.
I'm now hoping for a July release, and it too will support IPv6.

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TL;DR: Starting with v5, WP fail2ban will have full IPv6 support in every feature.