To help people migrating to 4.0.0 it is of value to be able to run the master in such a way that only certain environments use the setting "parser=future". The way it works now is to switch the entire master.
This may be as simple as just making the setting environment specific with a default value taken from the main section parser. However, there may be behavior that is conditional on Puppet[parser] that is sticky and the first task is to review those conditionals to ensure we do not have such sticky behavior.
Note, that when merging this up to master, the construct needs to be reverted since the parser setting is then removed.