How do you delete a thread that I started awhile back? I can’t figure how to do it. Thanks.

I don't know if this will work, but ...
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I assume that you were the OP (original poster) that started the thread?  Go to your post. Look for the "Report this post" link underneath your post. Click on that and tell TER Admin why you want the entire thread deleted. There is no guarantee that they will accept your reasons and delete it. But, you can give it a try.

even after it is after the initial 2 hour or so period where there's an option for it.

but the time limit to edit/delete a post on your own is 4 hours, not 2. AND whenever any post is deleted any post replying to it gets deleted too. In the case of an OP, that would of course mean that the entire thread would be deleted and YES, Admin does this all the time.

The edit feature is available for 4 hours (not 2 hours, mrfisher😉)  
After the edit window is gone, the only way to do it is to ask TER to do it. You can do that by using the “report this post”  button. You have to give TER the reason you want to remove it, and it is totally at their discretion whether they delete it or not.
The post is TER property once it is posted, and only they can decide to delete it or not.

Usually, if someone realizes they misspoke in an OP, they catch it fairly quickly and tender their request to TER admin to delete it, then you can let it go, or put back up an edited version and start over as a new thread.  However, removing a thread that was active for days or weeks is bad form.  Even though there were a few times when I put my foot in my mouth, I don't ever request a thread I started to be removed because I don't want to disrespect my fellow members who invested their own time to post responses and comments on the thread.  Of course, you're free to do what you want, but there are few members who became notorious post-pullers over the years and they eventually  have a hard time getting people to participate when they post, because everyone knows it might disappear the next day and so we don't want to waste our time.

Maybe someone else knows the answer to this suggestion. Can Admin EDIT the OP's OP? I think the answer is Yes.
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Sometimes, a Provider will append an image to a post on a "no images" Board. I think I have seen Admin leave the text but delete the photo. Did I imagine that or can they do that? (Maybe Admin deleted the entire post and the Provider quickly reposted w/o a photo.)
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Maybe your OP doesn't need COMPLETE removal and you just want to correct a major mistake. "So-and-so is a big, stupid moron." but you meant to say, "So-and-so is NOT a big, stupid moron." Or, "So-and-so cheated me out of my deposit." Oops, you meant to say, "So-and-so NEVER cheated me out of my deposit."
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Can Admin make such an edit? WILL Admin make such an edit (especially if you read CDL's reply: repliers replies will now make no sense)?

Back when this rule was actually enforced the most common edits of an OP were to delete a pic or link to a provider's website which by TER rules makes such an OP an "ad post" Rather than delete the post which would mean deleting the entire thread/threAD usually just the offending pic/link would be edited out. Of course lately those kind of ad posts seem to be permitted. Why? Only Admin knows and they aren't telling. lol

This is something that has been denied repeatedly, but it's common knowledge that Admin can and has monitored so called "private" conversations between members. I would suggest that if you have anything that you truly need to remain PRIVATE, do not have the conversation on any TER platform. This is hardly unique to TER btw, I would NEVER trust any DM on any platform to be truly private.

I still think my last board suspension, which was in 2019 while TER was dark in the US was prompted by a rather blunt assessment of the bias of mods we had at the time on the P & R board that I sent to you.   I actually stopped posting there for awhile until it went unmoderated because I felt there was no way I was going to be treated fairly at that time.  

Perhaps your PM was just too many words!  
I agree with GaG that TER can read PMs, and can edit them
If they wish, but I think it is very rare, especially as short staffed as they are. They can’t even read all the board posts and reviews. There is no way they read every PM. I think they would have to have a really good reason to read yours and especially to edit it. In your case, I would t be surprised at all if it were simply your word count.

Especially chopping "half of what you sent to the recipient"

the most likely scenario.  I have not tested the limits myself on how many characters a PM or a board post is allowed, but it is always something I'm aware of when writing reviews, so it makes sense that there may be a limit on other text boxes as well.