I think as long as these extra packs don’t replace work done by the devs on staff, it’s fine. Here are some things to consider:

• While it’s not specifically gig work, many studios already hire contractors to work on games while in development, just to terminate the contracts after launch.
• If these content creator and modder packs stopped being sold, they won’t automatically equate to content being produced by the studio instead. The staff may be working on other DLC or expansions, after-launch support, or moved to another project.
• The studio may decide to stop officially developing for a game, but community-made packs could be a way for them to add additional content.

I used to play Warframe, and the studio, DE, would regularly add skins that were designed and voted on by the community. The studio still produced their own cosmetics, so it was cool to have additional skins and also support designers in the community. Ultimately, I think we should look at these on a case-by-case basis, as we’ll probably see some studios doing it “right”, while others may lean a little too heavily on modder work.