Multiplayer requires latest version, so no getting updates results in losing multiplayer. I agree refunds would be a decent move given that multiplayer is the main selling point of the game though.

It's unrealistic to continue supporting multiplayer if you're not updating the client anymore. It'd mean you could never make changes.

I think you're missing the problem for the consumer.

If I bought that game, because it means I could play multiplayer with my friends and now they are removing that feature <3 months after I bought the game, then I would not be a happy camper.

I don't think anyone truly expects a game's multiplayer should last forever, but it should last while the game is still popular and releasing updates.

Totally reasonable to be unhappy, sadly it's a common occurrence in this era of live services. Sometimes you buy an online-only game and the servers go down a month later. If you're lucky the devs will be nice and refund you, sometimes they don't.