I am supposed to attend a workshop closely related to my research and as title shows, should I attend or not?

I gather from your previous posts that your relationship with your previous supervisor ended acrimoniously? I'm not sure of the details. Please provide some extra details, if you think they're relevant.

Do you think seeing him/her would cause you significant distress? If so, don't go. No workshop is worth triggering some seriously negative emotions. You're allowed to give yourself time to recover.

However, is this going to be a recurring problem, i.e. will your ex-supervisor be at so many events that it impedes your ability to attend important workshops, conferences, etc? If you think you can stand it, it might be better to go to the event and participate but avoid direct confrontation.

How big is the workshop? I've attended small-ish workshops (around 20 people) and still didn't speak to everybody over 2-3 days.

When is the workshop and have you already confirmed your attendance? If it is soon, dropping out might make you look unprofessional (though obviously, allowances have to be made for sudden illnesses, emergencies, etc.). Whether you've paid for transport, accommodation, etc. might also factor into your decision (but it shouldn't be more important than your mental health).

I would attend it. You have moved on to a better PhD role and there is no point hiding from him. If you do bump into him and have a professional conversation about what you are doing, it will give you closure.