15.2 doesn't fix the problem, or it doesn't retroactively fix the problem and I doubt it fixes future problems (i.e., where a volume stops indexing...again).
It's 3 jan 2025 and I'm debugging my issues with a 3-volume RAID, an external TB box with 6 drives (and 6 volumes), and the internal and external SSDs.
A couple minutes ago, I ran mdutil -as to see if the posted status would give me useful info. All volumes except /Preboot (and a network drive which we'll ignore) have the status of "read-only". But the man page doesn't say what this means, so it kind of looks okay? Nor does the man page tell you what you could get back, so I don't know what's normal.
One other thing that I have not seen in this thread IS from the man page, and that is "indexing will start/restart upon mounting a drive". So for those cases where the volume in questions is not your boot drive, you can unmount and remount to get indexing started. Like in my case.
To experiment with one of the suggestions, I went into Terminal and ran sudo mdutil -Ei off <volume> and picked one of my 6 drives. I also unchecked all "search results" boxes in the Spotlight System Settings. Then I entered sudo mdutil -Ei on <volume>. (Note that, yes, I did not need to include "E" again.) Then I hit cmd-spacebar to see if there was a progress bar: there was, but it was fully blue so I thought it was almost done--which I didn't believe. Then I remembered the System Settings and I checked all the things I wanted indexed. Ran mdutil -as and the one drive now had the status Indexing enabled. Checked the Spotlight results (cmd-spacebar) and now the progress bar was like 5%.
But like others, I have had volumes or even just folders stop indexing, and dragging into the Privacy box and out again doesn't seem to help, or be persistent. I have tons of bad aliases that are not easily removed because you have to click each one and select "Delete alias..." which it doesn't do immediately like you'd expect. Hopefully, running mdutil -E (etc) will, starting from scratch.
Finally, the man page says "indexing may be delayed due to low disk space or other conditions." But it doesn't say what other conditions could exist. It also doesn't inform you HOW to check if low disk space is the current condition. It should NOT be the case for all of my volumes...but might be on a 10TB drive with 6-9GB free?