I would bet money that there will not be a XF 3.x version for quite awhile (I'd say 2028 at the absolute earliest). This is of course with zero inside knowledge. But let's look at the facts:
1.0 was released in March, 2011
2.0 was released in November 2017
1.x had minor version released of: 1.1, 1.2, 1.3, 1.4 and 1.5
It's been 2 years since 2.2 was released.
Developers have said there's a 2.3 version coming (see
this post).
Developers have also said that 2.x have a longer shelf life than 1.x did because it's more mature. It doesn't need a complete/major restructuring or rewrite.
...so my logic is that is 2.x minor versions have a 2 year shelf life and we'll probably also see 2.4 and 2.5 someday... There's not going to be a XenForo 3 in 2023. Which is great as far as I'm concerned... Addon developers aren't into rewriting everything from scratch every time there's a major version. I'd be stoked if there was NEVER a 3.0 version. Let's just go to 2.99.