The snap keyword has been replaced by prime.
Snapcraft supports providing a list of files to include/exclude from two steps in the lifecycle: stage and prime. That’s done with two keywords in the yaml, respectively: stage and snap. The snap keyword has now been deprecated and replaced with prime, corresponding to the actual lifecycle step to which it applies. All other functionality remains the same. Where you wrote:
# ...
parts:
foo:
plugin: nil
snap: [bar]
Now write:
# ...
parts:
foo:
plugin: nil
prime: [bar]
See Deprecation notices for further announcements.