When I learned about taxonomy in the 90's they hadn't really sequenced many genomes, so taxonomy was still very much phenotype driven, rather than the modern genetic/molecular approaches. I just assumed that everything I learned has become out of date.

I learned Kids Playing Catch On Freeways Get Splattered about 20 years ago and I still remember, despite never needing to use it. So, the mnemonic helps to remember, but remembering doesn't really help me

Depends on the mnemonic. It has to flow and you have to know the steps individually. Although there was one time I spent so long trying to come up with a good mnemonic for the citric acid cycle that I just learned it and gave up.

Archaea and Everything Else (I can't remember the other domain)

The kingdoms have changed several times, they're basically trying to use a more accurate system based on DNA using genotyping to decide which groups are more closely related and be less reliant on behavior and appearance.

Stuff being separated away makes sense I guess