For those that play on Mirror, and those that build there too, buildmode is back. It should now work as it did before, including giving you some buffs, setting the time to day, healing you when you get damaged, and stopping you seeing any harmful things such as mobs or projectiles.
Note that, for those who are new to buildmode, it has rules. You should only use it if you are actually building, do not use it to “spectate” in pvp or to mess with a zombies gamemode. You will get banned. Make sure you read the rules if you haven’t already.