Cheating in Legacy has never been safe and will probably never be safe.
You need to take a few things into consideration if you want to add Legacy. Most of these things you aren't able to change and need to have already.
- Your account activation date. This is the equivalent to the date in which you purchased Dead by Daylight. You can go into Steam Account Details, Licenses and Activations to get this date. Do not confuse this date with your other Dead by Daylight DLC's. The Legacy time period was from Aug to Nov 24th 2016. Your account should be before, or during that date.
- Your account hours. You should have at least 100 hours.
- Your DLC. Back when Legacy was still available to get legitimately, The Last Breath Chapter (Nurse and Nea DLC) was a downloadable DLC. Meaning it would show up in your DLC library for DBD. Now The Last Breath comes pre-installed with DBD and there is no way to get it added back in.
- Your prestige achievements. Look at when you unlocked your Prestige 1,2 and 3 achievements. If you don't have them unlocked, unfortunately you wont be able to add them without the dates being this year. And this is also what everyone normally looks at when you wear legacy, if the dates are from this year or have no dates at all they will report you for cheating and if your profile is private (some) will also assume you're cheating and report you. It's a lose-lose situation if you don't already have the dates.
- Your prestige dates. On your save file, every character that has normal or legacy prestige has a date associated with each prestige. Fortunately, you can change these. You do have to be careful though, at around the release of the Nurse (and Nea), is when the devs implemented these prestige dates. If you have an account with an activation date before that date, and want to Legacy Prestige other characters besides the Nurse and nea, you can simply use Null's Save Editor to add Legacy. This will add a date from the year 0001. If you have an account activation date after the release of The Nurse, or would like to Legacy Prestige the nurse you will have to manually implement dates from that time period.
As you can see, if you weren't there to unlock Legacy legitimately, you really can't without asking for your account to get banned. Best is to play without Legacy.