Branch 'main' became 3.13.0a0 as of 3.12.0 beta 1: 2023-05-22
However, unless an patch is not backported to 3.12.b#, idlelib/News3.txt items continue going under
What's New in IDLE 3.12.0 on both main and 3.12 until 3.12.0rc?, 2023-08 or so. (None for 3.12.0b_.)
Subsequent news items go under
What's New in IDLE 3.13.0 (new header) on main branch
What's New in IDLE 3.12.z (new header) on 3.12 branch
What's New in IDLE 3.11.z (existing) on 3.11 branch
In other words, idlelib News3.txt is handled as if main were branched off as of about 3.x.0rc1.
This is different from the changelog attached to What's New in 3.x.

Release peps -- needed for proposed and actual release dates.
3.11 PEP-664
3.12 PEP-693
3.13 PEP-719

Linked PRs

• gh-112953: Rename idlelib/NEWS.txt to News3.txt and update #112988
• [3.12] gh-112953: Rename idlelib/NEWS.txt to News3.txt and update (GH-112988) #112990
• [3.11] gh-112953: Rename idlelib/NEWS.txt to News3.txt and update (GH… #112992
• gh-112953: Update news to finish 3.13 as main #129877
• [3.13] gh-112953: Update news to finish 3.13 as main (GH-129877) #129878
• [3.12] gh-112953: Update news to finish 3.13 as main (GH-129877) #129879