As I continue paging through Deminhumans of the Realms, I've noted a couple elf subraces that were not addressed yet in 5e, namely the Lythari lycanthrope elf and the winged elf. Are they playable in 5e? Well, they certainly need to be modified from their 2e baseline if we are to make them somewhat balanced for PC use. Second edition was not consistent at all in balancing races and classes, and it probably only got worse toward the end of the run when this particular splatbook came out. So these subraces are a strange mix of unusually strong bonuses and hindrances, for the Lythari including an aversion to armor but all the advantages and disadvantages of lycanthropy. Maybe we can dial some of that back a bit, as well as simplify what gets a little over complicated in the 2e description. Elves are also kind of tough to do the subraces for, as the Player's Handbook elves have the base elf doing a lot and the subraces somewhat less. So I think I'll need to dump the ability modifier as even limited shape-shifting can be pretty powerful; for the Lythari, I'll use base elf for all the elf stuff and use the shape-shifting ability as the thing setting them apart from all the other elves and leave out other modifications from base elf while in elf form but build more features into the wolf form.
Elf Subrace
Lythari
You are one of a rare breed normally found only in the deepest of elven forests. You are comfortable in both your elven and wolf skins, and happiest running with your pack through the forest. You are tall for an elf, typically 6' tall, with silver, gray, or white hair and pale skin. In wolf form, your coat is the same color as your hair in your elf form. Unlike many elves, you are a carnivore, and constantly seek out the freshest meat, eating cooked meat and plant based foods only as a last resort. Anyone who sees you transform may become fearful or hostile due to fear of lycanthropy.
Wolf Form
You have the ability to shapeshift into the form of a wolf as an action. In wolf form you can use an action to change back into your elf form. You lose your normal physical abilities and adopt those of the wolf form, except for your hit points and hit dice, which remain the same as in your elf form, other than as modified by the (Con) modifier of the wolf form, if different from that of your elf form. Your mental abilities (Int, Wis, Cha) and skill proficiencies as a wolf remain those of the elf form, and you also gain the skills of the wolf if you don't normally have them in elf form, and you use your normal proficiency bonus with the skill. You can't cast spells in wolf form. Both forms are natural for you, if you die in either you remain in that form after death. When you transform, you clothes and equipment do not change with you. A prudent Lythari removes clothes and armor prior to changing form. Clothing worn during the transformation is destroyed. Armor interferes with the change, and as well as being wrecked in the transformation it does damage to you, equaling one hit point for every point of AC it provides above 10, with magical armor doubling the damage done. As a wolf you connect best with your lycanthropic heritage, and are resistant to non-magical, non-silvered bludgeoning, piercing, and slashing damage. During the nights of the full moon, you are compelled to take your wolf form and cannot change into an elf until sunrise.
At 5th level your bite attacks as a wolf count as magical.