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.
Only the 27" model has the access panel, the 21" model doesn't. I'm not actually sure if the RAM in the smaller one is soldered on or if it even has additional slots. But even if it does replacing it isn't something you can do easily.
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.
The current 2020/2018 (they are the same) Mac mini does use DIMMs, but it is not easy to entirely disassemble the computer to replace the RAM. Some (2014?) Mac minis didn't have replaceable memory at all. The 27" "regular" iMac is still using a design from late 2012 that has a simple door behind the stand hinge to very easily access the RAM. Even the iMac Pro has to be totally disassembled to change RAM!
The post-lamp iMac has always offered user-replaceable SODIMM slots in a number sufficient to occupy all of the CPU's memory channels. The late 2009 model had 4 on the bottom, the 2015 has 2 one the back, these new ones have 2 and the Pro model has 4 (because the Pro CPU has 4 channels).
what? Since when did they offer a DIY upgrade path?