So I deleted all the EXIF on my photo, but found out I left out the IPTC data because never heard of it until I actually saw it still showed a ''date taken'' under the IPTC tab of software (Exifer) I use. If I upload my photo to someplace that is able to read/detect the EXIF, will they be able to see the IPTC data as well?
If the place you upload can only read EXIF data, then they wouldn't be able to see the IPTC data. But it is extremely unlikely that the site would be only able to read just EXIF data. Odds are it could read the IPTC data as well as any XMP data embedded in the file. Google Photos and Flickr, for example, will read EXIF, ITPC, and XMP metadata.