I discovered this the other day, and forgot that I had not posted about it. A change in merge behavior probably occurred with 23.4, but I did not test until 23.5 (and the newest Beta versions). With all the changes in the introduction of Text-based editing, I did not notice it. But when you discover it doesn't work as before, it stops you dead in your tracks. I found it a simple adjustment, and is now more powerful.
Merge no longer becomes active simply by clicking in a second segment. You must select part of at least 2 segments. But you can select multiple segments. For example, if you select the end of Segment 2, all of Segment 3, and the beginning of Segment 4, and then you click merge, it merges all of Segments 2, 3, and 4 together.
Split works as before. But now if you select a bunch of text, it will still split, but still at the cursor position. If you drag from end to beginning of selection, the cursor is at the beginning; if beginning to end, at end. If the cursor position is not a valid split (already at the beginning of a segment), split is not available.
Merge/split are not shown when in sequence view with a source transcript. Can you not do merge/split operations in text-based transcript in sequence view? I don't see how. You go to source monitor view of the source transcript to do split/merge. You can still create a static transcript of the sequence, and split/merge there, but that transcript will not update as you edit.
For captions, you must select two or more adjacent caption segments to get merge to appear. You cannot drag-select in the caption tab. I don't believe you could merge more than 2 previously, but I am not sure about that.
Split appears to work the same way as before, but my notes are not clear. Place the playhead over a caption in the timeline. Click split. It will duplicate the caption text, with the new timecode at the playhead. If a new segment is less than a second, it splits it in the middle. If you designate the caption to split by clicking in the captions tab/text panel, it positions the playhead at the beginning of the segment and splits it in the middle.
Stan