Back in #2490420: EntityAutocomplete element settings allows sql injection and for arbitrary user-supplied data to be passed into unserialize() → , settings started being stored in key-value and although there was a bit of discussion there about doing expiration, going for the less complicated method of having no expiration was chosen in order to more quickly release the fix.
Fast forward to present day. 82% of the data in our key_value table is the entity_autocomplete collection. We're talking millions upon millions of rows (gigabytes of information), most of which will never be touched again and will never be cleaned up. And this is only a 2-year old website where virtually all of the traffic is authenticated.
Permanent retention of this data is not sustainable. Even if a relatively high threshold is chosen for the expiry, it would still be preferable to keeping them in perpetuity.
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