While you cannot place solid blocks at level 255, it's possible to place various non-solid blocks. Repeaters and comparators can be placed, but cannot be adjusted.
Linked issues
duplicates 1
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It is also impossible to place minecarts on tracks that you've placed at the topmost layer, and impossible to switch the the position of levers, depending on what angle you're standing/floating at. Basically it would appear that most anything that requires right clicking is blocked at that level, except for placing some non-solid blocks.
When you attempt to adjust the repeater or comparator, it says "Height limit for building is 256", as though you just tried to place a block.