Why am I seeing ads in Instagram for searches in Safari? App tracking is turned off

While denying tracking requests in your settings is a good start, there are unfortunately many different ways to track people, several of which are not affected by such settings. Also of note, app tracking restrictions specifically mention sharing with "other companies". Since Facebook (now Meta) is a very large company, they can collect your information and use it for in-house advertising services, without technically having to involve any "other companies" at all.

If you would still like to restrict Instagram's tracking ability, I would recommend checking out their privacy center, specifically the section on exercising your rights. Here's a direct link.