Halo 3: ODST remastered is due to be released as DLC for Halo: The Master Chief Collection but no date was confirmed by 343 Industries. However Xbox.com was seen listing it for a May 29th release in the US and the UK. Trying to access this page again forwards you to the "Page not found" page, assuming it has been taken down until an official announcement. In December 343 announced they were going to give away the ODST package for free to anyone who played Halo: The Master Chief Collection between launch day and December 19th, 2014. This was due to matchmaking issues and bugs which resulted in gamers not being able to access the multiplayer. Further details were shown including a 8.1 GB file size for the game. No word has been said on the price tag for people who didn't play MCC during that period and will need to pay for the content. Any further updates will be listed here. [Update]
343 Industries has recently commented on the leaked launch date posted on Xbox.com. They stated that the date was merely a placeholder and apologise for any confusion that was caused:
"A placeholder release date for the ODST campaign for Halo: The Master Chief Collection was inadvertently listed on the Xbox Store." (1/2) "We continue final testing and will make an official announcement once our release date is determined, and apologize for any confusion. (2/2)"
The tweets state that they are in the final stages of testing which could only mean a release date is close by. Originally slated for a May 2015 release, and with that placeholder date slipping by, eyes are looking at the upcoming E3 2015 when a launch date could possibly be announced.