35-year-old man wants his girlfriend, who is 39, to end her friendship with her male best friend of 20 years. What are your thoughts?

I'm a 39-year-old woman currently in a relationship with my boyfriend, who is 35. We've been together for nine months, but recently we've had a lot of disagreements due to his concerns about my male best friend, who I've known for nearly 20 years. My boyfriend went through my phone and found some messages from 2017 that he interprets as flirty. In one exchange, I sent a photo of my Halloween costume, and the conversation included comments like "Hot" and "Garnet still would’ve been better though." He then asked if I got any numbers, to which I replied, "Meh nah." He also made a joke about the guys where I live, saying, "Your cleavage was too much for them." My boyfriend feels these messages are inappropriate and doesn't believe me when I say my friendship with him is purely platonic. He wants me to sever ties with my best friend, who is married and lives in my home country. I understand that he can be a bit loose with his words, but I assure him there's no flirting involved. I've also deleted all contacts of my exes and agreed not to travel home without my boyfriend in the future, but he still insists on cutting off my long-term friendship. He's been reading posts that label having a straight male best friend as a potential red flag, leading him to believe that there’s a risk involved. However, my best friend and I have maintained a platonic relationship for all these years, and I don't feel that the messages he found reflect anything inappropriate. What steps can we take to resolve this situation? Do you think the messages are flirty?