I saw both greek and ancient-greek come up when writing a question, and I couldn't figure out what difference there would be using one or the other, considering that we specify that questions concerning Greek must be about the time period "late antiquity or older" to be in scope.
I would be in favor of merging the two into one
ancient-greek tag. This way it is clear that modern Greek is squarely off limits. While there may not be any confusion now over the topic, leaving it without a modifier leaves it open to questions in later down the road. Changing it now will make when there are relatively so few questions means that we are saving ourselves time in case there ever is a need to change it later. It's future-proofing.
I would prefer to merge the two into
greek. It's by far the more popular tag (417 questions compared to 17), it's more concise and elegant, and as far as I'm aware we haven't been seeing much confusion about whether Modern Greek is in scope.
EDIT: The issue of
koine-greek was brought up in the comments; I see no issue with
greek being applied to the same question, much like how a question can be tagged with both