Expectation-wise, I've had it all over the place from just saying you like someone and them agreeing was seen as an agreement to go exclusive to those where until you agree to date (rather than just seeing each other) then as long you're not trying to hide/lie about anything, then exclusiveness is not expected.
As a teenager, I was encouraged to be more the latter - not to get too serious with one person, which suited me as long as things are clear - but met several of the former who were after that instant fairy tale that got nasty when I couldn't read their minds.
I also found the full spectrum of these in online dating back in the '90s, but it was a very different space to today - it was more people on the same hobby forums/chatrooms chatting privately deciding they liked each other and then would be 'seeing' each other. Again, largely high school/college/uni students, so in person and online, there was quite a bit 'hanging out' with people in the group.
That's how I met my spouse - online writing and roleplay forum. We had no expectation of exclusiveness -- it was kinda expected that when thousands of miles apart, we might date others and I was at the time Neither of us started dating new people and eventually all the others fell away. We didn't really have 'the talk', but it kinda naturally evolved into just us two when we started talking about living together for a summer (which turned into marriage 6 months later).