Apologies for the long post and this is my just guess so I could be wrong but...

I don't think it's SkyTeam limiting Google, but rather something fundamental has changed in the way Google Flights displays pricing and it's causing problems.

I have been getting the "Book with a travel agent" messaging across carriers (AA, B6, and DL off the top my head) and across different itineraries.

I believe Google Flights gets its pricing data directly from ATPCO (the industry clearinghouse for fares) and it does not know how to interpret the wide range of things that are possible to code for restrictions. When it runs into something it doesn't know how to handle, it throws up the "Book with a travel agent" message and a price that is probably not legal (from a rules perspective) to sell, but exists and can be sold on other dates or flights or whatnot.

Why does it say "Book with travel agent"? Well, I think it's a bit cheeky. A travel agent could sell this fare by force pricing it in the GDS. However, they will 99.9999% get a debit memo from DL and so it's not worth doing it.

If you are seeing it across DL, AF, and KL and widespread lately, the JV may have changed the way it did some of their fare rules coding, which is causing Google Flights widespread problems (likely something in a General Rule to be technical).