I’ve only heard of rare instances where EX’s have maintained a constant close friendship and carried it on through the years. I don’t personally know anyone who has done this but I can tell it’s not common. It may be common for those who: briefly dated, felt platonic for one another, became incompatible, or dated eons ago and reconnected later in life. Situations where I know a friendship for sure can never be a true constant platonic friendship (meaning you carry on a friendship with this person on a regular basis and they are in your circle of friends) is if the break-up ended badly, one or both persons still care for the other, it was a long-term relationship, you were married, one or both persons did something horrible to the other (cheated, betrayal, manipulation, abuse, lies, theft, etc.).
I see way too many people trying to build a friendship with a newly appointed EX and I know, it’s just not a good idea. Not now at least. After a break-up you need to give some time and space to each other to regroup and find your own individual selves again. The longer the relationship was, the longer you need. In my book, it’s at least 6 months to a year. If you try to be friends right away, one or both can very well get hurt. Sooner or later one person is going to start dating and you may or may not be ready to face that when it happens. Best to distance yourself and give yourself time to heal so that when you do face it, it won’t hurt as much.
Personally, I don’t suggest trying to be friends with your EX at all. You can remain cordial, and say hello when you run into them in public. But, it’s just such a sticky matter… there are so many people on this earth to be friends with why make things complicated? If you still at all care for your EX, being friends is a HUGE NO NO. You are just setting yourself up for pain and disaster. If your EX is the one who is still carrying a torch for you, then all the more reason to let it go. I know this person may have been a huge part or your life, your best friend even, but it’s best to make a clean break and find who you are apart from them rather than let the past linger around your future.