It is a concern that this could simply reflect changing naming conventions for private funds. There is nothing that requires a fund to use the "Fund I" convention.

Would it be possible to confirm the trend using Form ADV instead of Form D filings?

Form ADV is the form used to register an investment advisor, which is fundamentally different than disclosing a fundraising event. It could definitely be interested to look into. The SEC presents its data in a relatively simple format. Here is the link for Form ADV historical filing data: https://www.sec.gov/foia-services/frequently-requested-docum...

I just wanted to follow up and say I was confidently wrong previously. Thanks for posting, and for the idea.