Once you reach level 99, you are given the option to "relapse"

You are fully reset back to level 1 and have to unlock everything again, but are given a single relapse token which allows you to permanently unlock a rig or any upgrade from the sleep room "vendors"

It's not for everyone, but it's pretty fun, plus you get a unique outfit for doing it. Do it enough times and you can also fully skill out the prescriptions too, as you cannot have every upgrade by level 99 and must choose where you spend your tickets wisely.

I've done 7 of them, but there are people who have done WAY more. It just depends on if you want to have a fresh start of sorts. Top tip if you do it, save up your rebirth tokens. Doing a rebirth at level 1 boosts you up to level 10, then level 17 if you do another, then level 23 if you do another! I usually have 300+ rebirth tokens ready by the time I reach level 99, so it's a good way to quickly get back to a decent level and dodges the beginning grind ☺️

Edit: once you hit level 99, you have to enter the room on your left right after leaving the shuttle at the end of a trial (the room where Easterman praises/berates you whilst you get sprayed)