ae471fa978589fb61ecb0f89bbfe4d43cf2d5eac intended to deprecate the --keyserver option for gpg way back in 2.1.9. The stated preference on the gnupg-devel mailing list is to use the keyserver option for dirmngr. and it's not strictly necessary today because gpg ships with a reasonable default keysever.
But g10/keyserver.c contains a warning message "(use option --keyserver)" in some circumstances.
This should be cleaned up if we're serious about deprecating gpg --keyserver.
Note that T4466 also refers to cleaning up the documentation about --keyserver in gpg(1).