It was announced back in February, with the reason being that they spend a lot of time simply preparing releases, rather than actually working on improvements to Mint:
Releasing often is important because it means we get a lot of feedback and bug reports when we introduce changes. We follow the same process over and over again. It’s a process that works very well and it produces these incremental improvements release after release. But it takes a lot of time, and it caps our ambition when it comes to development. With a release every six months plus LMDE, we spend more time testing, fixing, and releasing than developing. https://blog.linuxmint.com/?p=4991
The latest blog-post suggests that their new schedule is already paying off:
Going from a 6-month release schedule to a yearly one was a tough decision to make, but we’re really starting to see it pay off. We’re taking on projects and implementing changes which would previously have been considered too large or ambitious. We’ve got time. We don’t feel rushed. Development feels solid and the OS keeps getting better. https://blog.linuxmint.com/?p=5086
For anyone curious about the number of bugs introduced in each kernel release, LWN.net has a plot of Bugs fixed and introduced in each kernel release.
However, it has the obvious limitation that recent releases will appear to have fewer commits introducing bugs, simply because kernel developers haven’t had as much time to find and fix the bugs introduced in those releases. So it’ll be a while before we can actually see the consequences (positive or negative) of more widespread LLM usage by kernel developers