What? It's literally in the title of RFC 5233. "Sieve Email Filtering: Subaddress Extension". The introduction starts with a sentence that defines subaddressing

What? It's literally in the title of RFC 5233. "Sieve Email Filtering: Subaddress Extension". The introduction starts with a sentence that defines subaddressing. It has half a dozen references to using + as the separator between the username and the "detail".