These days the common format of Garmin maps is a gmapsupp.img for each map. These files can be used as they are on the devices and QMapShack. There is really no reason to waste more time on that crazy *tdb stuff.
I disagree, to me it would not be a waste of time. Many people use QLandkarte GT to display OpenMtbMaps on their PC. Such maps are in Garmin format, but you don't get a single gmapsupp.img file. You get a whole set of small .img tiles, and a mapsetc.tdb file.
Removing .tdb support from QMapShack means you need to still rely on QLandkarte GT to produce the consolidated gmapsupp.img file for QMapShack. It's true there are also other tools to do this job, but they are command line tools like mkgmap or gmt--not a viable option for the average user I believe.
Furthermore, personally I am a registered member of OpenMtbMap, and I'm allowed to download the complete Europe map. I can't produce a single large gmapsupp.img file for all Europe and load this huge file on my Garmin device. Therefore I use QLandkarte GT to select a sub-map (i.e. just the tiles I need), and create a gmapsupp.img file that contains just the data from those selected tiles. I upload such much smaller .img file to my Garmin device.
Since QMapShack won't support tdb files, you don't have a choice and have to rely on QLandKarte GT to create submaps. In other words, you have to rely on an old piece of software that is no longer actively developed, and this might mean such a useful feature will no longer be supported in the future. I believe this would a real pain.