I had no 100% confirmation. However, it’s already “legacy”, the legacy option isn’t available on Discourse-hosted instances (at least the first tiers) and I think it will be discontinued an probably removed at some point.
Can’t say for sure, but I think you should be able to disable it with a plugin… Thing is, you would ensure that even if the sidebar doesn’t show up, it doesn’t make unnecessary requests.
Now it’s far beyond my knowledge! 