@Lola4321 thank you for the quote about the split venues. I have, honestly, never come across that as I said in fifty plus years of attending weddings. But each to their own.
I do understand where you're coming from. If you don't live in a country you can see their ways as odd.
As someone who's attended many evening bits of a reception as a friend, I'd find it very hurtful if I lived in Australia and wasn't invited to a friend's wedding. But I now understand that's the norm for that country - day guests only - and they can't afford everyone.
And that's it really isn't it? What's normal for that country. Someone on here has already said they've been in Canada for some years and they do the same as the UK (day guests and evening guests).
For our family and friends group we see it as a lovely way of seeing all our friends whilst doing the obligatory 'auntie Mary' invitation to the ceremony. We'd be very hurt if we missed a chance to celebrate with our newly wed friends - day or evening.
A lot of venues, similarly to our venue, do a wedding package which includes evening buffet. You wouldn't get a discount if you just had day guests, the food would be put out in the evening and taken away at the end if it wasn't eaten. Most day guests don't touch apart from, maybe, a slice of pizza at 10pm! Why not invite a few extra (think 30 in our case) to enjoy five hours of partying with a big buffet?
It's horses for courses and I don't really care what the rest of the world thinks about our funny little ways. Most countries have funny little ways in their customs from my point of view and I can't be bothered getting worked up about them. As long as I understand that something is the custom of the country I'm in, I go with it. I certainly wouldn't say it was caste or class related in those countries and if that's what the world thinks of us I won't be crying into my wine. They do theirs and I do ours.
Maybe things need to be done differently here and in many countries, that's why we do have so many mixtures of 'right way to have a wedding' in this country. Some have a huge reception (evening included), some just have a pub meal with their immediate family. Some picnic, some have a sit down meal, some have a tea party. My cousin, back in the 1960s, had a registry office wedding, went off to see their favourite football team play then had just an evening party with everyone turning up (even if they hadn't attended the wedding). Each to their own. It's an invitation, not a summons and I, you and everyone else can decline if they want to - most of us wouldn't be missed to be honest.
The issue is that the OP is feeling hurt and that's a shame and is probably unnecessary as the B&G could have sorted out a compromise. But, if having her (as someone I'd not met) to the main wedding means someone in the family can't come then I'd explain to her DP that I'd have to invite the two to the evening part rather than split them.
And maybe they already have had that conversation unbeknownst to the OP?? She doesn't mention how her DP has reacted to this - if he's not ranging from a bit miffed to outraged, maybe he already knew what the compromise was? If he's not too bothered, maybe he's agreed it?
However she does say that, in 2.5 years, she's not met the bride and groom. Which I find odd as we've had restrictions lifted for a year now in the UK. In fact we went to an evening part of a local (1.5 hours drive away) wedding reception last August and a wedding 400 miles away at the end of last year. So not meeting half way last summer for example, seems very strange if you're best friends and getting married soon.
I know she says they live a long way away so maybe the B & G live overseas and are just back for the wedding with family over here. Even then though, I find it strange that the OP's DP hasn't even arranged a couple of zoom calls (unless I missed that). I'd be wanting to at least zoom meet with my DP's best friend. Very odd but maybe men are like that?