+1 prof. as the only gain besides hp over two levels is not much, imo. YMMV and all that
You'll find only two classes really have any "dead" levels. The Barbarian at 15th and 19th and the Fighter at 19th.
For example, the cleric at level 11-20
11: 6th level spells
12: ASI
13: 7th level spells, Proficiency Bonus +5
14: Destroy Undead, Proficiency
15: 8th level spells
16: ASI
17: 9th level spells, Proficiency Bonus +6
18: Proficiency
19: Extra 6th level slot
20: ASI, Extra 7th level slot
Additionally, every level offers the Cleric another prepared spell to choose.
While some levels might seem weaker than others in terms of what you gain, there are no (actual) dead levels at all for the Cleric. You are always gaining
something besides HP.
Now, due to higher level spells, ASI, and Proficiency, I think most (if not all) caster classes have no dead levels.
What about non-casters?
Due to the combination of ASIs (12, 16, 20), Proficiency (14, 18), and Proficiency Bonus increasing (13, 17) the only possible dead levels are really just levels 15 and 19.
Barbarian: Gains nothing at those levels. But, I could easily bump up rage damage to +4 at 15th instead of 16th and then just 19th would be a dead level.
Fighter: Gets Extra Attack (3) at 15th, so only 19th is dead.
Monk: Continues to gain Ki at levels 15 and 19, so no dead at all.
Paladin: Gains a 3rd level spell slot at 15th and a 5th level slot at 19th. No dead levels.
Ranger: Gains a 3rd level spell slot at 15th and a 5th level slot at 19th plus new known spells at those levels. Nothing dead there.
Rogue: Sneak attack increases at both levels 15 and 19. So, no dead levels.
So, really, in total, the Barbarian has two dead levels, which I could easily fix to one, and the Fighter only has one. Every other class gains
something besides HP at every level.