I started Halo:Reach legendary in season 5, but when I try to resume now, it crashes and brings me back to the main menu screen
They said that mid-level checkpoints pre-Season 6 would become unusable after the update drops.
Here, from Postum’s post about it:
First, mid-mission checkpoints for all games in MCC will not be valid post-update. You will still have access to play the campaign missions but will need to restart from the beginning as the previous ones will become invalid.
Sorry, but nothing else to do but start it over again.
Hope this helps!