Airtable has started to enforce record limits by preventing people from adding new records after an unspecified grace period.
Since you are actively using the base and adding data, I recommend taking steps to get under the record count limits immediately. Although Airtable will not delete any of your existing data, it sounds like being unable to add data will be a problem.
Depending on your current plan and budget, one option is to upgrade your workspace to increase your record limits.
Other options options involve duplicating the base to archive old data, and then deleting old data in the active base.
Still other options involved “flattening” data in linked records (often using a script) and then deleting linked data.
