Welcome to the discussion forum, Rhys_H!
Yes. The Arahanthood is the ultimate achievement, which is the “perfect state”, once attained lasts forever, per Buddha.
I am not sure how familiar you are with Buddha Dhamma, the Theravada version. Even the Theravada version has been contaminated over the past 1500 years or so, and thus there may not have been that many Arahants in recent years, especially among humans (there could be more in other higher realms).
I discuss this in the post:”Incorrect Thēravada Interpretations – Historical Timeline“.
There has been a revival in recent years due to my Noble teacher, who passed away a year ago, likely as an Arahant: “Parinibbāna of Waharaka Thēro“.
There are indications that there are many people now with at least lower stages of Nibbana (Sotapanna,etc), thanks to his efforts.
Also see the posts in the section: “Nibbana“.