The Meta-Chlorians just need to do their job a little better

I can't speak to the number of iPhones in active use that can't be updated, but the iPhone 6s, released in Sept 2015, still gets iOS 14 updates, presumably at least until the next major update, which should put it around 6 years of updates.

Facebook isn't equivalent... you are either messaging existing friends within Facebook or people from their post. You do have the option to upload contacts to Facebook to add friends that way, which is then part of your account, which FB messenger uses. Which is a roundabout way of saying that FB Messenger wouldn't mind if you uploaded your contacts to Facebook.

Whatsapp.... it asks for your contacts
Telegram.... same
Snapchat.... oh yeah
Nimses... these guys too
LinkedIn.....of course (to be fair, not specifically a messaging app)
Hangouts... yup
GroupMe.... mmhmmm
Skype.... +1

So yeah, maybe your experience has been different than mine and you use an entirely different set of messaging apps, but asking to use contacts is the norm, in case sharing of that contact information makes it easy to identify people that you already know that use the app. So yeah, you might want to review your app permissions, but it's fairly likely that regardless of whether you allowed it, accessing your contacts was asked for.