The IRS Guidance says this in 5.05(2), which is most relevant to software startups:

  (2) Computer software developed for sale or licensing to others. In the case of
  computer software that is developed for sale or licensing to others (or upgrades 
  and enhancements to such software), activities that occur after such software (or 
  upgrades and enhancements to such software) is ready for sale or licensing to 
  others, such as marketing and promotional activities, maintenance activities that 
  do not give rise to upgrades and enhancements, distribution activities (for 
  example, making the software available via remote access), and customer support 
  activities.

So they are maintenance as long as they "do not give rise to upgrades and enhancements", which would be the responsibility of the taxpayer to track. I'm sure there is more nuance to it in practice.