The Eldritch Invocation "Eldritch Smite" says the following:
Prerequisite: 5th level, Pact of the Blade feature
Once per turn when you hit a creature with your pact weapon, you can expend a warlock spell slot to deal an extra 1d8 force damage to the target, plus another 1d8 per level of the spell slot, and you can knock the target prone if it is Huge or smaller.
Unlike a paladin's Divine Smite, which says "to a maximum of 5d8", Eldritch Smite says no such thing, so does that mean that a Warlock 5/Sorcerer 15 can use an 8th level spell slot to deal 9d8 damage using Eldritch Smite?
Related: Can a multiclassed warlock expend a Spellcasting spell slot of a non-warlock class to use the Eldritch Smite invocation? (this was found by MikeQ, and I added it here after asking this; this related Q&A address a core mistake in my question, but I'll leave my question here for others who overlook the same thing I did...)