Until these happen -if ever-, I have no reason to be happy about any of this. Even if we get these rules, this is all coming out of the erasure of half-elves and half-orcs. Having an explicit option that doesn't reek of blood purity language would be an improvement, but still we are dealing with the de-normalization and marginalization of mixed characters.
I see the playtest normalizing mixed-parentage characters. The "Half-Elf" and the "Half-Orc" are no longer weird. Now mixing is something that all sapient species can and do.
In the earlier playtest, there seems an over-reliance on customizing appearance and flavor, because there are concerns about mechanical balance if unintended "combos" become possible.
But if I recall correctly, the designers recognize there also needs to be a mechanical way to represent a character of mixed parentage.
For me the obvious solution is to handle traits in the same way that 5e handles feats.
Say every official Species has three choices from a list of half-feats. (Something like Flight might cost all three half-feat choices.) These are called "Special" Feats (= Species).
For example, below is moreorless the UA playtest Elf, but organizing into three Special Feats to choose from, namely Nature Spirit, Fey Ancestry, and Twilighter.
ELF TRAITS
Creature Type: Fey or an other Creature Type of your choice
Size: Medium (5-6 feet tall)
Speed: 30 feet
Lifespan. You reach adulthood about age 20, like Humans do, but afterward remain eternally youthful. Due to other causes of death, Elves expect to live 750 years on average.
Choose three from the following Elven Special Feats.
NATURE SPIRIT
Choose two cantrips from any spell lists. You can also treat the following effects as if cantrips when you choose:
• your speed improves to 35 feet
• you can breathe water as well as air
• you can after each long rest swap a cantrip with an other one of your choice
• if you have Darkvision it improves to 120 feet
The two chosen cantrips are innate, thus you cast them without spell components. Intelligence, Wisdom, or Charisma is your spellcasting ability for them.
FEY ANCESTRY
Charm Resistance. You have advantage on saving throws you make to avoid or end the Charmed Condition on yourself.
Innate Caster. From any spell list, choose a slot 1 spell when you reach level 3, and a slot 2 spell when level 5. You gain the ability to cast each of these two spells once after a long rest. You know these spells and can also cast them using any Spell Slots you have. The spells are innate, thus you can cast them without spell components. Intelligence, Wisdom, or Charisma is your spellcasting ability for them.
TWILIGHTER
Trance. You dont need to sleep, and magic cant put to you sleep. You can finish a Long Rest in 4 hours if you spend those hours in a trancelike meditation during which you retain consciousness.
Darkvision. You have Darkvision with a range of 60 feet.
Keen Senses. You have Proficiency in the Perception Skill.
The three above Feats are flexible enough to represent almost every "elf subrace" in the history of D&D. There can also be other Elf Special Feats to choose from, such as Flight, or Magic Resistance, or whatever. More powerful Special Feats can become available at higher levels, such as becoming an Archfey. Meanwhile, the Species chapter has a section that explains what to do when choosing Special Feats for a character from mixed parentage.