A few months ago I found out that another member of staff, who is in a different team to me but sits near us, has organised a regular meal out. I think a while ago it was only a few people who went but it has evolved over time. It has now got to the point where everyone in my team is invited except me.
OP, I'm sorry you are so affected by this.
It sounds, from your account, more like a situation that evolved, rather than a deliberate exclusion - a person outside the team started it, it was only a few people initially, big over time it's grown to include everyone.
This is thoughtless but unlikely intentional. Yes, you'd expect someone to pay more attention, and ensure you were included.
The key question is: what do you want to happen? If you want to go, you should say so, via the organiser or your boss.
It isn't great behaviour from them; but it's also perplexing that you are so lacking in assertiveness you have sat there saying nothing while they discuss an event! And your only suggestion is to ask to wfh on these days to avoid the discussion.
Gently, I expect your own lack of self-esteem has put you on the periphery. It's time to speak up, and if the response is ti continue to exclude you, that's a diff effect conversation with your boss.