We will see if this remains the rule, that was just the UA material. I understand the reasons, but it makes it not backwards compatible. I don't care that people say there is a kloogy way to do it - it's not backwards compatible to eliminate one of the most popular races, in both name and mechanics, and pretend it's still compatible. So we will see if they received that message or not.
I think the 2024 Players Handbook can thread the needle. Use the Human-Elf as the main example for how to build a multispecies character, and make sure it is reasonably satisfying.
Here are the 2014 Half-Elf traits. The Ability bonuses are now moot. Language is moot, 2024 grants three.
• Fey, Charm Resistance
• Darkvision
• Choose two skills
In my opinion the above three traits are unworthy of defining a species. In any case.
Here, is the 2024 Elf traits
• Fey, Charm Resistance
• Darkvision
• Perception skill proficiency
• Cantrip plus cantrip-like lineage
• Slot 1 and slot 2 spells
• Trance, Sleep Immunity
Allowing any character to swap a cantrip for any proficiency would then approximate the 2014 Half-Elf. Indeed, there can be a new cantrip that itself grants a proficiency with skill, tool, weapon, or language, swappable per Long Rest.
In any case, even Fey, Charm Resistance, and Perception skill is already close enough, especially when the background and background feat can grant many skill proficiencies.
Here is the 2024 Human traits.
• Extra Inspiration
• Choose one skill proficiency
• Free background feat
Indeed, a new 2024 background feat that grants Fey, Charm Resistance, one skill proficiency, and Darkvision, would be spot on for the 2014 Half Elf, but isnt even worth doing. To make it more appealing, grant Fey, Charm Resistance, Darkvision, and one cantrip.
(Again, the Darkvision itself can be a cantrip, and absent from the 2024 Elf description but available as a choice of cantrip. In which case, the elven Feat would be Fey, Charm Resistance, plus choose two cantrips, which feels elven enough when using a Human stat.)
The 2014 Half-Elf lacks enough information. There is little or nothing to rescue.