Long post, sorry:
Nothing was ever officially announced but it was hinted at. Megalo, the game type editor for Reach seems to point towards some sort of post release patch though. The whole development of Megalo was intended provide post game support for Reach. They tested it with Halo chess and some of the other gametypes that fell under insane category in the gametype list. It seems highly unlikely that they would have spent so much dev time on Megalo to not actually use it. Megalo was developed specifically to make patches like 343's Reach TU possible. With the way the Halo engine worked before Reach changes like zero bloom would not have been possible without directly editing the .map files which was not really something they could have done. So they had developed the tools to patch the game post launch but they never used it which leads me to believe something happened post launch that made them change their plans. The megalo Q&A seemed, to me at least, to be Bungie hinting at their desire to make a TU and the scope that the TU could have encompassed. So while I say they planned a TU I mean they had planned to do one before the game was even released but it looks like something to do with the upcoming transition stopped them from doing so. We can only guess as to what happened but the most plausible answer is that 343 told Bungie to provide playlist support but not patch support so that 343 could patch Reach for the sole purpose of CEA.
-snip-
Edit:
That still doesn't make much sense though. If it was so easy, then why did Bungie ever do it? 6 months isn't a short time. Bungie did promise everyone that they would be supporting Reach as much as the other titles.
Like I said, I would believe that Bungie may have planned a TU if they ever talked about it in any serious way like 343 is now with the TU in February. That never happened though and it was ignored. The lack of support in Reach should be on Bungie's shoulders. 343 released a TU that saved MLG Halo for a limited time, even though it could have been better, it was something.
Also, Bungie released a TU for Halo 2 & 3 a few months post launch. Time is a thing that can't really be ignored on this. If Bungie was just "in talks" about a TU when 343 took over, that's embarrassing in itself.
I just thing most of the blame for the lack of support in Reach needs to go to Bungie, not 343.
For the same reason everyone seems to know the Didact's name.
343: if you didn't read the books, you're a scrub and don't deserve to know lol get better
