RAM on iMac's has been user upgrade-able since at least 2014. I have a 2014 27" iMac and a 2017 27" iMac, and I upgraded the RAM on both of them. It's not a crazy "disassemble half the machine" type job either, it is really easily done via a panel in the back.
iMacs have always had upgradeable RAM, just about the only user-serviceable part in them. There's a little door you can open on the back by the power connector. https://support.apple.com/en-us/HT201191
Haven't the iMac and Mac Mini always offered (relatively) easy memory upgrades? I've upgraded probably 10-15 of them at work. They just use the samller laptop modules instead of a desktop module, and (usually?) only support 2 modules. Mac Pro offers it too, though that's a different style of computer entirely.
