I was doing a little research on Epic Boons and post-Level 20 advancements. It appears that Warlocks are not eligible for one of the most powerful (and most popular) Epic Boons: Boon of High Magic.
Boon of High Magic from DMG page 323: You gain one 9th-level spell slot, provided that you already have one.
Since the Warlock's 9th Level Mystic Arcanum is cast w/o a spell slot, I'm assuming that the underlined text above prohibits your not-so-friendly neighborhood warlock from taking this boon.
I guess Boon of Spell Recall is another way to get that extra 9th (or 8th, or 7th, ...) level spell:
Boon of Spell Recall: You can cast any spell you know or have prepared without expending a spell slot. Once you do so, you can't use this boon again until you finish a long rest.
I was doing a little research on Epic Boons and post-Level 20 advancements. It appears that Warlocks are not eligible for one of the most powerful (and most popular) Epic Boons: Boon of High Magic.
Boon of High Magic from DMG page 323: You gain one 9th-level spell slot, provided that you already have one.
Since the Warlock's 9th Level Mystic Arcanum is cast w/o a spell slot, I'm assuming that the underlined text above prohibits your not-so-friendly neighborhood warlock from taking this boon.
I guess Boon of Spell Recall is another way to get that extra 9th (or 8th, or 7th, ...) level spell:
Boon of Spell Recall: You can cast any spell you know or have prepared without expending a spell slot. Once you do so, you can't use this boon again until you finish a long rest.
Am I interpreting these Boons correctly?
I think you are interpreting these boons correctly. Mystic Arcanum has nothing to do with any feature based on spell slots.