Do high level multiclass casters lose spell slots?

[Scurvy]

A level 19 wizard has three 9th level spells per day. A level 20 multiclass spellcaster gets one 9th level spell slot. Does that mean if a level 19 wizard multiclasses into wizard 19/cleric 1, it goes from 3 a day to 1 a day and loses spell slots?

[bekeleven]

A level 20 multiclass spellcaster gets one 9th level spell slot.

Where is this from?

[Scurvy]

Where is this from?

From 5e apparently. Searching "multiclass spellcasting 3.5" prioritized 5e results first and I didn't notice d20srd has 5e info now.

[Biffoniacus_Furiou]

In 3.5 edition you use the spellcasting progression of each of your classes separately to determine your spell slots per day for each class.

In 3.5 edition full spellcasters get multiple 9th level spell slots per day in the higher levels. In 5e, no spellcaster gets more than one 8th or 9th level spell slot per day, and only get a second 6th and 7th level spell slot at 19th and 20th level. The 5e multiclass spell slot table is the same spells/day of each level as a single-class full caster's spell slot table.

5e is the only edition in which multiclass spellcasters have a shared pool of spell slots. Previous editions including 3.5 track spell slots for each class separately.