As a rule of thumb, anyone that he may meet in the future or anyone that you interact on a regular basis, like a coworker, he should be informed about. Seeing a one night stand from 3 years ago in the grocery store need not be mentioned, even if they recognize you and you have a 5 minute chat. If they want to meet up for coffee or whatever then again, mention it to your significant other. This removes all possibility that he be surprised by someone else mentioning you fucked someone he knows or you still speak to. This gives him all the information he needs to know before deciding if he can’t handle you speaking to or interacting with these people.

Many people will say it’s none of his business and you’re not lying by omission by not telling him but that really doesn’t matter. It doesn’t. In reality he is likely to feel fucked up if he’s caught off guard with this info and you will have to deal with not only that, but him also feeling like he is 100% justified for feeling how he feels. Get ahead of this and deal with it from an advantageous position now.

Completely agree. My current SO for the last 8 years recently lied to me about a long time friend of hers and the true nature of their past relationship. While their current friendship might have been innocent, the fact that she deliberately lied to me about their past when she would talk to me about him has caused damage to our relationship that I'm still not sure can be repaired.