Should We Still Install Matched Pairs of RAM?
No - even in the core 2 days, it was almost always faster to have more RAM that wasn’t matched compared to less RAM that was matched. I believe the penalty was 3% range - so you could test it in a benchmark, but real world - more RAM wins.

The only way matched ram would matter was in a perfect tie. Say two chips of 4 compared to four chips of 2. Always go with more RAM when you have capacity unless you are on a power budget - mobile devices on battery have to power all RAM and if you provision too much, you waste battery. 

These days, the memory controllers are so good, I don’t think you can even benchmark differences due to a mix of memory sizes or locations.