[popinman322]

PCA is a foundational dimension reduction technique, and kNN can be used in conjunction with embeddings.

k-means is still great when you have prior/domain knowledge about the number of groups.

[nephanth]

K-means is pretty poor when the clusters are not linearly separated, but it is the basis of a lot of more modern clustering techniques (kernel K-means if you have prior knoweledge, spectral clustering...)