It is very standard to not remove or reassign note numbers on engineering drawing revisions.. In general, the removed note is simply replaced with "REMOVED" or equivalent. OBSOLETE = REMOVED in my mind. Also, some organizations use the cross-out method thus leaving the original note.
In aerospace they work to MIL-DTL-31000 standard which outlines the TDP including the engineering drawing practices. Clearly, it is a good idea to NOT re-number as this effort could introduce drawing errors and create extra labor and more spend.
Be aware that many organizations have internal DRM's and may choose any practice that best fits their organizations requirements.
I would check out the following standards:
ASME Y14,35M - Revision of Engineering Drawings and Associated Documents
If you work in aerospace, see Global Engineering Documents "Drawing Requirements Manual" as you probably live by this document.
MIL-DTL-31000 D -->mil-dtl-31000.pdf