I know what you’re getting at but I think you’re wrong to suggest it’s to do with grammar. Putting oneself after others in a list is stylistically better but not to do so isn’t incorrect.

Both are arguably correct but me is preferable. It can only be ‘my boyfriend and I’ if one is trying to say ‘this is my boyfriend and I’, accepting that the verb ‘to be’ takes the nominative case. But I presume you don’t usually say ‘this is I’ because that sounds like an affectation.

You would never say this is a picture of I. So you mustn’t say ‘this is a picture of my boyfriend and I.’

As far as I’m concerned it isn’t a throwback; it’s simply the correct spelling. 😊

Try: My boyfriend and me in our Hallowe’en costumes.