While applying updates, an additional logger UPDATED_ENTITY_IDS should catch updated entity IDs (one ID per log line), this logger by default is configured to collect entity IDs in gzipped plain text files with per-day rolling policy, limiting size to not exceed 10G (that is a guess value, and should be configured for production according to actual updates load and available disk space).
This updated entity IDs log will be useful while researching issues with updates and also provide a reliable and easy way to identify entities which were recently changed (see also task for testing of Merging Updater T231411)