I too am very concerned about this question. As a longtime Gimp user, I can tell this is not a Gimp issue. Even 2.6 and even former versions would keep EXIF without problems.
To me, the most serious issue is that TIFF files sent directly to Gimp by RawTherapee are without EXIF, even if ‘Copy Exif/IPTC/XMP unchanged to output file’ is checked.
I have used RawTherapee for years. Generally, as I really want to keep the EXIF in my xcf files, and I don’t want to edit an already altered JPEG file in Gimp, I export two files: a JPEG and a TIFF output, open the JPEG with Gimp, then add the TIFF as new layer in order to keep the processed image as lossless as possible, then delete or overwrite both output files;). Using Exiftool to transfer metadata from the raw file to the output is another workaround, but both are very messy and slow.
I don’t like Adobe products at all, but Lightroom can send to Gimp in PSD format so that metadata are kept and the image is not compressed with a lossy algorithm… RawTherapee developers, do you want to make me happy? Please, add such a feature, (maybe using xcf)!