On the elementary.io website I see this logo
The logo is a SVG file (I had to o convert it) and uses a custom R and Y letter. There is also the boot logo which is
Is the first one a custom font or a font. And is the other one another font
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From the brand section at elementary OS webpage:
It is a meticulously-designed brand mark, not simply "elementary" written in a specific font.
More info: elementary.io Brand