I saved a single toolbar in 1.20.5 and then opened it back up in 1.20.4 to find that it's completely empty. I would think that it would only affect that one toolbar instead of the entire sheet. I suggest that data versions should be assigned for each row of items individually, and also have 1.20.4 compatible versions of the items saved as well (if possible). I haven't looked too much into the file structure of the hotbar.nbt to tell if it's possible for older versions to have the new changes. I also suggest that you could either reload the saved toolbars in game, or have multiple sheets of toolbars that can each have their own data version, in case you want to work in multiple versions without losing your precious items.

edit:
Looking at the nbt in both files, it looks like one could store both "components" and "tag" in the item field, and have both "count" and "Count" assigned at the same time. I tested if i can load all 4 tags and it seems in 1.20.4 the item shows correctly with the tag and Count, and in 1.20.5, the item shows correctly with the components and count tags. So I suggest at the very least, when the data version updates from 3700, it saves ALL 4 of the components in the hotbar.nbt.