Some of the blocks I had saved in my toolbars don't get overtaken, namely concrete blocks and also wool in my case.
Some of the toolbars that contained only concrete or wool are completely empty and thus "ready to be saved over".
Also, different spawn eggs I had in my player inventory vanished as well.
Also, chests as well as enderchests are also affected.
For example, if you stored a shulkerbox which contained spawn eggs in an enderchest, then that shulkerbox won't contain those spawn eggs anymore.
Spawn eggs in item frames of course also vanish.
I currently assume this includes any kind of block entity who contained an affected block or item, thus also e.g. probably droppers, hoppers etc. pp.