What's this orange container thing? Unable to rearrange individual pages.

You have a word processing format document. What you get by default. The only way to get page 4 content before page 2 would be to take these following steps:

1. Place your insertion point in front of the first character on page 2, and from the Insert menu > Page break. Now you have a blank page preceding what was page 2.
2. Select all of the content on the former page 4 and cut it to the clipboard. Click on that blank page 2, and then paste into it. Your original page 4 is now blank.
3. Click in the first text position of your now empty page 4, and press backspace to remove that page entirely.

You need to understand that you are using the word processing format of Pages and nothing has changed with regard to limitations with section/page thumbnails. You can only rearrange sections and their accompanying pages, and there is no deletion of a page by attempting to delete a thumbnail. You would need to select the entire contents of a page and backspace twice to remove it and its preceding page break.

A Pages Layout document type is where every thumbnail is both a section and a page, and these can be arbitrarily rearranged, or specifically removed.

See Pages User Guide for Mac - Apple Support

And if you a tired of that orange treatment in Pages, then visit System Preferences > General > Accent color on Monterey or earlier, or Settings > Appearance > Accent color on Ventura.

The only thing that changed is the type of document you were using last month. What you describe cannot be done with a word processing document then or now, but can with a page layout document.

You are welcome.