There is no reduction in the official number of spaces being carried on the Norwich to London services. They have always had reservations, and guards have had a lot of flexibility to allow people on. When the new trains come in by the end of next year there will be no guards van, thus the rules will be enforced. In the run up to the new trains Greater Anglia are getting people used to this new enforcement that will have to happen due to less space in the bike area of the new trains. Any frustrations about the reduced bike space should be directed to the DfT and their requirements for new train franchises. The DfT will likely say that the increase in seats to cater for future demand and peak times is a frequently requested requirement, hence it be being a priority. When the DfT consults on rail franchises, more cyclists need to respond to the consultations to ask for more cycle space.

The new trains will all have 6 bike spaces, thus the other services into London from Essex, and on the West Anglia route which don’t have any cycle space at the moment.

You may be interested in new rules the EU are introducing about the carriage of bikes on trains, re a minimum of 8 bikes per train for any new or refurbished train in 2 years time, though sadly won’t apply to the UK, unless something magical happens.
https://twitter.com/citycyclists/status/1063078651024031744
https://twitter.com/Al__S/status/1063079393524883457