Is Nihil still available in Warframe?

Solution 1:

Yes. It is still possible to fight Nihil the Glassmaker, and you still earn the sword he originally rewarded.

You have to complete Nightwave challenges to earn Nightwave standing; in turn, you can purchase Nightwave currency. You then need to purchase a "Nihil's Oubliette" decoration for 60 Nightwave currency, and a "Enter Nihil's Oubliette Key" for a further 60 Nightwave currency, at a total of 120 Nightwave currency.

After installing the "Nihil's Oubliette" in your Orbiter, you can use the keys to fight Nihil. Keys can also be traded, so you might be able to enlist the help of a friend, or otherwise trade the key for something else of value that you have.

According to the Warframe Wiki,

Acquisition

The Vitrica's blueprint is obtained after defeating Nihil in Nightwave: Series 3, Episode 5.

Following the end of Nightwave Series 3, Nihil's fight now requires an Enter Nihil's Oubliette Key and a Nihil's Oubliette decoration to access, both of which can be purchased from Nightwave Offerings for 60 [Nightwave Currency] each, and the former can be traded between players. Once the Oubliette decoration is installed in the player's own Orbiter, interacting with it with the Key in the player's inventory will trigger the boss fight with Nihil. - "Vitrica" @ Warframe Fandom