I think the most honest response would be "it depends". Many people will relapse back into anxiety if untreated and some will not, regardless of not being treated at all. Also, there are several distinct anxiety disorders (e.g. panic disorder, generalised anxiety disorder, social anxiety disorder) that differ in terms of how likely a relapse is with/without proper treatment. The evidence differs between mental health problems as to whether psychotherapy (mostly CBT for anxiety), medication or a combination of the two is the most promising approach with the least risk of relapse. According to evidence regarding social anxiety disorder for example, the improvements seem to stabilize and even continue growing AFTER psychotherapy/initial recovery (probably because people are more likely to keep generalising and extending their insights and changes in behaviour/cognitions to other interactions and areas in their lives).
Having suffered from social anxiety disorder for almost all of my teenage years and some remaining problems in my twenties (and not having received any treatment whatsoever), I've been fully recovered for over a decade now. I have had extensive CBT training, though, and was lucky to have made similar or the same experiences I would have made in therapy (which I totally should have given a try). While "sore spots" remain that sometimes remind me of how bad it was back then, personally I can say:
It does. get. better.
And aside from my personal experience, that's how clients describe the process of improvement as well. Treatment definitely enhances the likelihood of a sustainable recovery.
One of the basic ideas of cognitive behavioural therapy is that what makes you anxious probably is not exclusively found in your workplace (or any other relevant circumstances respectively), but is based on what you make of it mentally and how you react to that. If that has not intrinsically changed after leaving your former job, you might carry it to the next, more stressful or demanding job and risk a relapse. But that's not necessarily a given either.
Whatever kind of anxiety you've experienced and are afraid of going back to, maybe you can find more answers here, for example:
https://www.anxietynetwork.com/
I hope this helps! Best of luck and take care!