The idea of exes remaining in active communications even while in their respective new relationships is the biggest of red flags that even both of them still in active communication shouldn't overlook.

Imagine an ex not being loyal enough to his/her new relationship, and is looking to talk past memories with their ex that might lead to cheating their new partners in the process.

If you think about this, even after cheating on their new partners with you and somehow you get back with your ex, you will likely lack trust for them. Because they will likely do the same thing to you if you guys are back together with their new relationship partners they left because of reconnecting with you.

I'm very fine with an ex remaining totally an ex forever if they ever got into a new relationship after a breakup.

Never make up with an ex that have moved on with a new partner.