We know that people hit the limit, probably due to ResourceLoader, but we don't know how often that happens. Going forward, even if we move RL's cache out of localStorage, we need to have visibility on real world usage of localStorage. Ideally we would not only track when users hit the limit, but also get statistics on what is taking up the space.
I think this is particularly important as 3rd-party JS libraries or even gadgets could also be consuming localStorage space without us noticing.
As a quick start, we can add tracking of localStorage to asset-check. That'll give us insight into the default/deterministic use of localStorage on major wikis. We already track this for cookies.
This would account for all mw.loader.store data for modules loaded by default, and any values set by gadgets/scripts on load.
Getting RUM telemetry would even better since many values are only set later during real user interaction. Perhaps using EventLogging, or more light-weight using the existing statsv beacon (would need to extend wgWMEStatsdBaseUri/wikimediaEvents.statsd to support gauge in addition to timing and counter).
Browser stats can give you this information: https://browser-reports.wmflabs.org/#all-sites-by-os-and-browser
This task is about gathering data about how much of the available storage capacity is used. E.g. how many keys or how many bytes. It is not about whether a browser supports localStorage.