I want to use Coda to entirely replace OneNote. I would like Pen support so that I can draw anywhere in the doc, write math equations, take handwritten notes, etc.
I don’t even need the handwritten stuff to be searchable.
Please add this feature. There are web APIs for stylus/pen including tilt and pressure sensitivity, and Google has implemented Pen input in Google Keep
Pointer events - Web APIs | MDN (mozilla.org)
GitHub - amoshydra/draw: Web canvas that support pen pressure
Sample HTML canvas code.
The pressure curve on this is near perfect for handwriting or drawing. Don’t change it!
Open source MIT licensed code 
Unlike this canvas example, don’t let people draw with their finger mouse (just don’t handle those pointer events)
Canvas needs to be infinite length, meaning I should be scroll left/right/down anywhere and write.
I can settle for infinite canvas but vertical scrolling only if that’s easier to implement (width confined to width of the device or whatever the max width is for text I guess.
That would be really cool, I personally use Samsung Tablet with active pen for all my notetaking, and also for some collaborative software (like Miro).
But I don’t think its realistic to think this feature will be added to Coda anytime soon. When you take a look at feature set and polish one note taking app has, going back to my tablet - Samsung Notes, its highly unlikely people from Coda will have time to develop such thing in near future. Unless maybe you can “buy” whole technology and integrate it, but again some more advanced packages are probably very expensive (if we talk Samsung Notes level of polish, accuracy and feel of writing and added features - and there might be even better software’s out there).
I might be totally wrong also 
Try the repo I linked.
The amount of code to get this working with a smooth pressure curve is surprisingly little (and it’s open source).
Of course, the amount of effort it would take to integrate it into coda is a different matter.