I don't see any underused time period, all web novel writers are scrapping the barrel.
Are you talking in absolute numbers, or by percentage?
When 90% of all fantasy writers set their stories in a medeival European setting, then by definition all other settings are underused.
Another thing to consider on this note is the level of magic civilization. This is the idea that, while the technological development might be considerably behind that of modern day Earth, the development of magic is high enough to elevate the civilization to quite a significant level. Almost all fantasy web-novels have a civilization level that is somewhere in the early 1800s in terms of living standards, and mid 1900s in terms of health standards, despite being in the 1600s in terms of non-magical technology. All this civilization is accomplished due to magic and enchantments.
It is incredibly rare to have a fantasy setting where magic technology is not doing all that much to improve people's lives in leu of the technological development. I can only think of 3 off the top of my head. Middle Earth (from Lord of the Rings,) Greyhawk, Dragonlance, or any other medieval-set world in the Dungeons & Dragons format worlds. (Pathfinder society worlds have significantly more advanced magical technology.) And, the third is my new favorite webnovel, Mushuko Tensei.
Any setting I've seen other than those 3 has all kinds of things like mana-restoring potions, teleport gates, communication magic devices that are more expensive than telephones but serve the same purpose, and all kinds of other things that make life a lot more convenient than it is in the 3 settings I just laid out above.
EDIT: (And also, yes. None of those 3 settings I mentioned have a way to restore a mage's spell power other than just time. Denying them something like a mana restoration potion makes for a serious resource pressure that, in the case of D&D, can be used to put pressure on the player or in the case of Mushuko Tensei creates some interesting pressures to use in the plot since the MC never has the easy out of a mana restoration potion since they don't exist in that world at all.)
So, that's another matter of the setting I would say is under-utilized. An early magi-technology set magical world. You might also notice, the three I mentioned as using this kind of early magi-technology setting are among the highest rated in the fantasy genre. It takes real writing skill to purposefully toss away the convenience magic could provide and brutally tell the characters in your world "no, magic cannot solve your problems that easily." And, that skill is often reflected in the form of a marvelously well-written story.