NOR flash memory chips behave the same as other types of flash memory. If I energize NOR flash once a year, will it prevent any data loss?
The datasheet says retention 20 years. If I energize it after 19 years, will it prolong data retention?

From my personal point of view, developers usually assume that NOR flash does not loose it's values over the lifetime, so they do not implement any refreshing, so I guess that energizing them wont help. I think the best method would be to dump it, create checksums of the dumps and store backups of those dumps and the checksums. Then you should be able to recreate the contents of those NOR flashes in case of errors.

If I were manufacturer I would say, if you energize after 19 years, we only guaranteed one more year of data retention;)

Nice Micron datasheet about that.
https://web.pa.msu.edu/people/edmunds/D ... ention.pdf
All those characteristics are not Micron but JEDEC specification. Looking at the curves, the retention depends on memory wear level and temperature.