I think a 10-20 liter pack is the sweet spot for activities like outdoor festivals.
Enough room for gear and snacks, a water bottle and even a jacket.

Agreed. The one I use the most when outdoors is an older Camelbak that I think is around 12L. With the hydration bladder full, I have room for a trauma kit, a boo boo kit, a ham radio HT, PSP, knife, sunscreen, bug spray, small multitool and a few other odds and ends, with the main cargo compartment being about half-empty to any accept activity specific items.

The smallest regular backpacks I have are around 18 or 19L which is a nice size for a day pack. I don't do festivals, just hikes and camping trips. On a day hike I still like to have the minimum stuff I'd need for an overnight stay; to me that's pretty much the definition of a PSK.

Anything more than about 20-23L and I'd want a proper waist belt that takes some of the load, so a small bag like the Trident 20 is about perfect for day trips / overnighters or misc light work IMO.

Friends swear by this: https://www.rei.com/product/148592/rei-co-op-flash-18-pack

I just ordered this in 10L since it's on sale. I'm a big fan of their bomb-proof products!

https://www.goruck.com/rucksacks/bullet-ruck/

The Bullet is almost the only GoRuck bag I don't own. Very tempting at this price!

Comes in a variety of colors and prints and in a 22 l version with more features

Problem is....most 'festivals' now limit things and don't allow more than a woman's clutch size bag.

I went with the REI Flash 22. When I compared the two the slightly larger Flash 22 had three exterior pockets besides the main compartment which makes for better organization. Still lightweight and simple.

What? Why? How is one supposed to carry rain gear then if not a small back pack is allowed.

The main reason given by hosts is security. Small bags make it harder to sneak explosives and guns into the festival. Some even search the small bags before admitting people to the festival. Many festival hosts report how much the vistors accept the security measures. OTOH they complain about about the lack of attendance.
A festival I used to frequent lost me as a customer because of their measures.
As a motorcycle rider I use a 5.11 Rush 12. The backpack offers enough room, is small enough to not be in the way and is ready for using a water bladder.

People at events often ask me "Where did you get that umbrella?"
I brought it. In my small pack.

I have gotten that question a number of times. “Amazon.”

Add: I thin everyone who hikes should have their own backpack; easier access to water, redundancy and room for their own gear

One main advantage of a small pack on trip or going to events is space- There's space for a little battery/ charger, a coat and a snack.