In my opinion, it is a personal choice that people make and it is not my place to judge whether it is ethical or not.
However, before you start using medicine to do your job or to perform, you need to know that these drugs can be addictive in the long run. They don't create a physical addiction but they may cause you to always rely on them to play and you'll end up always needing a beta blocker to the point that you won't be able to play without drugs. You don't want that.
Do carpenters, dentists and lawyers need drugs to do their job? Of course not. So why should we musicians have to resort to this? If one person needs drugs to work, that person has a problem. If half the people in the profession are in this situation, perhaps we should ask ourself if this business isn't a bit too demanding or too unforgiving to the musicians or if we aren't asking too much from ourselves and our colleagues.
It takes time and patience to build the self confidence and mental toughness to be an orchestra musician. Most people will have ups and downs in their self confidence level in the course of a season or in their career. That is normal and expected. You can't be at 100% all the time. When I don't feel so well, I try to take a few steps back and take it easy for a while. I remind myself that I'm human and have the right to make mistakes. It usually makes me feel better rather quickly. It can be very tempting to resort to beta blockers when things don't go your way but I prefer trying to calm myself down by other means. At the end, there's nothing wrong in being a bit nervous sometimes. It's a normal human reaction. Even the greatest players get nervous sometimes. If there's nothing wrong with you, why taking pills?
To be honest, I'll admit that I have tried beta blockers for some auditions but never in concert. Not even solo recitals or concertos with orchestras. In a audition, the stakes are high and you only get one chance. It is a lot more stressful than an orchestra rehearsal or concert so for this particular occasion, I could understand that one would want to use medicine to calm his nerves. Having said that, I wouldn't do it now if I had to audition again. Drugs won't give you a better tone, make you play more in tune or be more musical. There are no pills for that! So I would just go in there and do my best. If they really like my playing, they can live with a bit of "vibrato". At the end, you can still play with confidence and sound great even if you are a bit nervous. Having sat in many audition committees, I can tell you that intonation, rhythm, tone quality, accuracy and musicality are the main factors that will determine if you advance or not. If you are strong in these five criteria, you most likely have enough self confidence to perform in stressful situations and don't need drugs to do well.
Taking medication to perform is a personal choice. Think about the consequences carefully before you make that choice. It may be appropriate in certain circumstances but you don't want to get hooked for life. It's up to you. It may be tempting to resort to drugs sometimes but I personally prefer the longer and more difficult path of building self confidence and mental toughness with time, patience and experience. In the long run, I appreciate my job better for it.