Use at own risk. Dev likely removed for some reason. works great for me so far though.

Instructions

cd <wtv the %Appdata%/Factorio/mods is for you>
# unzip attach-notes_0.3.1.zip (version may differ)
git clone https://github.com/321freddy/attach-notes.git
cd attach-notes
git revert ce711cfdb18fdd9d092cbb95a1f1e6deef823aab # the commit that removed this feature
cp scripts/entity-notes/controller.lua ../attach-notes_0.3.1/attach-notes_0.3.1/scripts/entity-notes/controller.lua 
cd ../attach-notes_0.3.1
# zip attach-notes_0.3.1 (the inner directory).
# name the zip file to wtv the outer directory name was (usually the same... in my case "attach-notes_0.3.1"... keep the ".zip" of course
# move the new zip file into the mods directory so that it replaces the old one

# boom... notes inside blueprints

PSA: If using this solution, dont use titles greater than exactly 25 characters when pasting signs. 25 is okay.
26 will crash your game.

BP feature far outweighs title character limit IMO.

Here is the provided stack from the error in case mod Dev wants to fix this.

The mod Attach Notes caused a non-recoverable error.
Please report this error to the mod author.

Error while running event attach-notes::on_built_entity (ID 6)
__attach-notes__/scripts/entity-notes/controller.lua:357: attempt to call a nil value
stack traceback:
    __attach-notes__/scripts/entity-notes/controller.lua:357: in function 'decodeNote'
    __attach-notes__/scripts/entity-notes/controller.lua:382: in function 'func'
    __attach-notes__/framework.lua:98: in function <__attach-notes__/framework.lua:97>

Similarly... a character limit of 75 characters exists for the note body. Same stack trace. that limit gets smaller when an icon or title is set on the sign. Not sure if its really a null pointer or potential memory limit imposed by game's architecture onto the sign entities representation within a blueprint. Occurs during decoding process,