[phasmantistes]

It's a good question! I know that our first root (ISRG Root X1) used that naming scheme simply because it was cross-signed by IdenTrust's root (DST Root CA X3) which used that same scheme. But where they got the scheme from, I don't know.

Using Y to denote the "next generation" of roots is a scheme I came up with in the past year while planning our YE/YR ceremony, so it's certainly not something that people were thinking about when they named the first roots.