If I don't prepare any Form 1040 returns but prepare returns that flow-through to Forms 1040 such as Schedules K-1 for Forms 1065 or 1120S, how do I answer the question, "Do you prepare Form 1040 series tax returns?" You should answer "no".

If you do not prepare any Form 1040 returns but do prepare returns that flow-through to Forms 1040, such as Schedules K-1 for Forms 1065 or 1120S, you should answer the question, "Do you prepare Form 1040 series tax returns?" with "no." This guidance is consistently provided across multiple IRS documents, including the IRS FAQ from 2023, Instructions for Form 1120-S (2023) from 2024, and Instructions for Form 1065 (2023) from 2024. Each of these documents explicitly states that if your tax preparation activities are limited to preparing returns that flow through to Form 1040, such as Schedules K-1 for Forms 1065 or 1120S, without directly preparing any Form 1040 returns, your response to whether you prepare Form 1040 series tax returns should be "no."

Sources:
Instructions for Form 1120-S (2023)
IRS FAQ
IRS FAQ
Publication 1380
Instructions for Form 1065 (2023)
IRS FAQ
Publication 17 (2023)