Seriously though, it's complicated...
On one hand we have hit points which are essentially linear for everyone.
We then have the Proficiency bonus, affecting attacks and spells, skills and saves. It is also linear but it's SLOW. See bounded accuracy.
Spellcasters daily slots also more or less linear, just a bit tumbling around the line. They start with 2, end up with over 20.
But then how about average damage per round, does it increase proportionately with level? I don't usually care for damage statistics, my gut feeling says it's less than linear but I'll leave it to the experts to enlighten us.
And then there is known spells, and here it's unclear. Wizards are linear, but Clerics and Druid get A LOT more spells at low levels, and then progressively less. This IMHO it's too complex for beginners. OTOH the range of utility from those spells is limited, especially from Clerics: many healing and defensive spells with different details don't really open up dramatically new tactics, not in the same way as some Wizard spells can.
So different aspects of character progression don't increase in the same way, and non-spellcasters rarely unlock more than one single new trick per level.
I do think that 5e is still not the best for beginners due to some excess complexity, but I think the problem is more at levels 1-3: races get too much all at once, and different spellcasting options are immediately thrown into the game. The most complex class at 1st level is IMHO the Cleric who at 1st level already has cantrips, bonus action spells, rituals, channel divinity, concentration spells, domain spells, and 15+ spells known... aargh!
What bothers me afterwards is the default speed. It's easy to change, but I don't like the general culture of speeding 1-20 in every campaign. It does feel unsatisfying for the story when this maps to just a couple of years in-characters at most. It makes you wonder why the PCs were stuck at level 1 all the years before the campaign, and what they are going to do all the years after...