Starting with Safari 27.x on macOS 27 the little + icon on the left of the URL bar will no longer add the current site to the Reading List, but instead it will put it where it thinks it belongs.
An example: I view a website about integration of a heat pump and the + adds it to my bookmark folder "House" in the Bookmarks Menu.
This seems to be an AI feature guessing where the bookmark would fit best. But it annoys the **** out of me, because in the past this would simply add the site to the Reading List. Now I always have to remember this. Countless times now I had to edit my bookmarks to remove the unwanted entries there.
Long-clicking lets you select the Reading List instead and it also still automatically goes there if the AI has no good suggestion, but this makes it all the more inconsistent.