Not entirely true. You can setup your experiment to be user keyed (as it should be) and can run it for a long time, or can run the hold back for a long time
Something as basic as mod(hash(experiment_group_id or layer id, userid), 1000) would give you a user stable experiment distribution that you can analzy long term impact on.
Long-term A/B testing has major problems in the modern world where users can communicate with each other (both onsite and out of band) and can use multiple accounts / devices / browsers
Something as basic as mod(hash(experiment_group_id or layer id, userid), 1000) would give you a user stable experiment distribution that you can analzy long term impact on.