WordPress › Support » list-more-custom-field-names
By default, WordPress only allows 30 custom field names/keys to be listed in the dropdown selection 'Name' field when writing a post or page. If you, or the plugins you use, make use of a lot of custom field key names, you may surpass the default limit resulting in some custom field keys not being listed. This may force post authors to manually type in custom field key names if they aren't listed, which increases the chances for naming errors (typos, or not using the precise key name that is needed). This may also cause some authors concern wondering where previously used custom field keys have gone since they aren't listed.
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