DMG p. 231-232 describes many epic boons for after level 20. Two of those are:
Boon of High Magic
You gain one 9th-level spell slot, provided that you already have one.
and
Boon of Spell Recall
You can cast any spell you know or have prepared without expending a spell slot. Once you do so, you cant use this boon again until you finish a long rest.
I'm not sure why would I pick High Magic over Spell Recall. Spell Recall
lets you cast a spell you know of any level (in other words, it gives you a spell slot of any level). High Magic gives me a single level 9 slot. As far as I know, you cannot gain level 9 slots with short rests, so it seems like one of the boons is just a narrower version of the other.
What are advantages of the Boon of High Magic over the Boon of Spell Recall?
Best Answer
High Magic has two advantages over Spell Recall.
Upcasting
The spell slot of High Magic can be used to cast any known spell at level 9. Many spells are indicated to have enhanced potency when cast with a greater level spell slot than required, commonly referred to as "upcasting". Spell Recall cannot upcast, as the extra effects are specifically worded to take place when the spell is cast "using a spell slot of Xth level or higher".
Features that use spell slots
Some class features allow non-spellcasting uses for spell slots. The Paladin, for instance, can invoke Divine Smite using a spell slot for extra damage (although they'd need to have another class that gives them access to a level 9 slot to gain High Magic). Judging by same reasonings as answers to this question I believe the level 9 slot from High Magic is eligible for use with Divine Smite, although the damage is still capped per the wording of the ability. Another example would be the Sorcerer, who can convert their spell slots into Sorcery points.