Are there any places in the world that maintain like 15-20°C year round?

Say around Oakland California really. It's about 500 miles north of San Diego.

San Diego is far warmer. I went to school in San Diego county. Heck even LA is far warmer further north.

Seen it as high as 35° c. (95°f) In San Diego. Far higher inland.

Close! Okinawans have it great, but there's variation outside that norm in each year.
British Columbia comes close, too, but with a month of colder days and a few weeks of heat in each year.

The pacific northwest is close to that on the west side of the Cascades. . In Tacoma the Average is about 12°. About the same in Seattle and Olympia.

Essaouira in Morocco has that kinda weather pretty much. It is often cool & breezy but can get quite windy at times

San Diego California is close to that on the coast. If gets a little warmer in the summer, but not much.

Canary Islands. San Diego. San Francisco . Parts of Morocco . All considered best weather locations in the world .
Subtract 5 degrees Celsius and you have Southern Ireland ,the pacific NW, etc...

Tbh thats an ideal temp for me. Not too hot. And hopefully nice and cool in shady areas etc. Anyway im here to see where those places are lol

Mid-latitudes in the northern and southern hemisphere would fit in that rough category.

Parts of the big island of Hawaii , based on elevation. On the Kona side.