Regarding my profile page, I only own Sims2 games, and no matter what I do, I'm not able to turn off ownership of Sims3 games. Ownership of Sims4 games is off, so that's correct, but Sims3 doesn't want to turn off.
How to fix?

Are you not able to toggle them here:
https://modthesims.info/profile.php?do=gamesowned

Thanks for the bug report! This should hopefully be fixed now.

I don't think this was a bug.
I failed to notice that each game has it's own 'Save" button. I had only noticed the 'Save' at the very bottom of the page, which only affects the Sims 4 selections.
So this was a "User Error'.
That said, I don't know why all three need their own 'Save' function, instead of just having a single 'Save' at the bottom of the page.

It was actually a bug. If you de-selected The Sims 2, for example, and hit Save, then it wouldn't save the fact that you didn't have Sims 2.

The reason it doesn't have a single Save button is because it's 3 seperate fields in the database.