Make it so that you never forget spells, but you have a hard limit on the number of spells you can know at any time (except possibly for Wizards; the limit could vary from role to role). Not specified if you could still learn new spells and forget old ones to make room like move sets in Pokemon, or if you’re locked into knowing the first N spells you learn in the game. It’s also OK for amnesia effects to cause spells to be forgotten.

Restore ability potions can (partially?) cure amnesia effects.

Getting amnesia can strip your knowledge of (and therefore ability to write) Elbereth.

When spells are forgotten or retention is damaged when you suffer amnesia, the game prints a message informing you of such, akin to “You forget some of your training” for skills.

Spectres should have some method of stealing your spell knowledge, then gaining the ability to cast those spells themselves.

A worn oilskin cloak protects you from the amnesia effects of Lethe water.

Amnesia makes you forget spells and lose skill points instead of forgetting discoveries and maps. Normally, spell recall is drained; if a spell is at 0%, it is completely forgotten.