Just experience, but my definition is pretty broad. Once you get out of the valley most of what pays well (banking, finance, telecom, analytics, industrial, etc.) is maintenance code IMO. Basically anything that doesn't come out real R&D budget, even if it is a "new feature", is maintenance to me at this point.

The caveat there is 'paid hours'. The current working model for the industry is that all software engineers leetcodeuberhack on open source repos at night and by day have paying jobs maintaining companies' systems that use open source.